Add CI tests with Java 8

[vlsi]

It looks like the recent org.ops4j.pax.url:pax-url-aether to v2.6.15 release breaks mvn:.. resolution.

A reproducer could be https://github.com/pgjdbc/pgjdbc at commit 45df56c7bc90fcecd184db4a9653319227c5facb.
The command line would be ./gradlew :pgjdbc-osgi-test:test -PjdkTestVersion=8

The error does not reproduce if running the test with Java 11+.

The error is as follows:

FAILURE   0,0sec, org.postgresql.test.osgi.DataSourceFactoryTest > classMethodactoryTest
    org.ops4j.pax.exam.TestContainerException: Problem starting test container.
        at org.ops4j.pax.exam.nat.internal.NativeTestContainer.start(NativeTestContainer.java:212)
        at org.ops4j.pax.exam.spi.reactors.EagerSingleStagedReactor.setUp(EagerSingleStagedReactor.java:86)
        at org.ops4j.pax.exam.spi.reactors.EagerSingleStagedReactor.beforeClass(EagerSingleStagedReactor.java:136)
        at org.ops4j.pax.exam.spi.reactors.ReactorManager.beforeClass(ReactorManager.java:457)
        at org.ops4j.pax.exam.junit.impl.ProbeRunner.run(ProbeRunner.java:97)
        at org.ops4j.pax.exam.junit.PaxExam.run(PaxExam.java:93)
        at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.runTestClass(JUnitTestClassExecutor.java:112)
        at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:58)
        at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:40)
        at org.gradle.api.internal.tasks.testing.junit.AbstractJUnitTestClassProcessor.processTestClass(AbstractJUnitTestClassProcessor.java:54)
        at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.processTestClass(SuiteTestClassProcessor.java:53)
        at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
        at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
        at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
        at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:92)
        at org.gradle.api.internal.tasks.testing.worker.TestWorker$2.run(TestWorker.java:183)
        at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:132)
        at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:103)
        at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:63)
        at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)
        at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:121)
        at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:71)
        at worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)
        at worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74)
        Caused by: org.osgi.framework.BundleException: Unable to cache bundle: link:classpath:META-INF/links/org.ops4j.pax.exam.link
            at org.apache.felix.framework.Felix.installBundle(Felix.java:3258)
            at org.apache.felix.framework.BundleContextImpl.installBundle(BundleContextImpl.java:147)
            at org.apache.felix.framework.BundleContextImpl.installBundle(BundleContextImpl.java:120)
            at org.ops4j.pax.exam.nat.internal.NativeTestContainer.installAndStartBundles(NativeTestContainer.java:340)
            at org.ops4j.pax.exam.nat.internal.NativeTestContainer.start(NativeTestContainer.java:209)
            ... 23 more
            Caused by: java.lang.IllegalStateException: Stream handler unavailable due to: null
                at org.apache.felix.framework.URLHandlersStreamHandlerProxy.openConnection(URLHandlersStreamHandlerProxy.java:300)
                at java.net.URL.openConnection(URL.java:1027)
                at org.apache.felix.framework.util.SecureAction.openURLConnection(SecureAction.java:777)
                at org.apache.felix.framework.cache.JarRevision.initialize(JarRevision.java:150)
                at org.apache.felix.framework.cache.JarRevision.<init>(JarRevision.java:76)
                at org.apache.felix.framework.cache.BundleArchive.createRevisionFromLocation(BundleArchive.java:819)
                at org.apache.felix.framework.cache.BundleArchive.reviseInternal(BundleArchive.java:491)
                at org.apache.felix.framework.cache.BundleArchive.<init>(BundleArchive.java:155)
                at org.apache.felix.framework.cache.BundleCache.create(BundleCache.java:457)
                at org.apache.felix.framework.Felix.installBundle(Felix.java:3254)
                ... 27 more
                Caused by: java.lang.reflect.InvocationTargetException
                    at org.apache.felix.framework.URLHandlersStreamHandlerProxy.openConnection(URLHandlersStreamHandlerProxy.java:292)
                    ... 36 more
                    Caused by: java.net.MalformedURLException: Unknown protocol: mvn
                        at java.net.URL.<init>(URL.java:645)
                        at java.net.URL.<init>(URL.java:508)
                        at java.net.URL.<init>(URL.java:457)
                        at org.ops4j.pax.url.link.internal.Activator.createConnection(Activator.java:100)
                        at org.ops4j.pax.url.link.Handler.openConnection(Handler.java:43)
                        ... 37 more
                        Caused by: java.lang.IllegalStateException: Unknown protocol: mvn
                            at org.apache.felix.framework.URLHandlersStreamHandlerProxy.parseURL(URLHandlersStreamHandlerProxy.java:362)
                            at java.net.URL.<init>(URL.java:640)
                            ... 41 more