feat: add close method to UpdateChecker

Executes shutdownNow() on it's executor service

src/main/java/com/github/khakers/modmailviewer/UpdateChecker.java

@@ -7,6 +7,7 @@
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+import java.io.Closeable;
 import java.io.IOException;
 import java.time.Instant;
 import java.util.Optional;
@@ -15,7 +16,7 @@
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 
-public class UpdateChecker {
+public class UpdateChecker implements AutoCloseable {
 
     private static final Logger logger = LogManager.getLogger();
 
@@ -49,6 +50,7 @@ public UpdateChecker() {
 
     }
 
+
     public static boolean isContainerized() {
         return System.getProperty("ENV_TYPE").equalsIgnoreCase("containerized");
     }
@@ -132,5 +134,53 @@ public boolean isSemVerUpdateAvailable(String version) {
     }
 
 
+    /**
+     * Closes this resource, relinquishing any underlying resources.
+     * This method is invoked automatically on objects managed by the
+     * {@code try}-with-resources statement.
+     *
+     * @throws Exception if this resource cannot be closed
+     * @apiNote While this interface method is declared to throw {@code
+     * Exception}, implementers are <em>strongly</em> encouraged to
+     * declare concrete implementations of the {@code close} method to
+     * throw more specific exceptions, or to throw no exception at all
+     * if the close operation cannot fail.
+     *
+     * <p> Cases where the close operation may fail require careful
+     * attention by implementers. It is strongly advised to relinquish
+     * the underlying resources and to internally <em>mark</em> the
+     * resource as closed, prior to throwing the exception. The {@code
+     * close} method is unlikely to be invoked more than once and so
+     * this ensures that the resources are released in a timely manner.
+     * Furthermore it reduces problems that could arise when the resource
+     * wraps, or is wrapped, by another resource.
+     *
+     * <p><em>Implementers of this interface are also strongly advised
+     * to not have the {@code close} method throw {@link
+     * InterruptedException}.</em>
+     * <p>
+     * This exception interacts with a thread's interrupted status,
+     * and runtime misbehavior is likely to occur if an {@code
+     * InterruptedException} is {@linkplain Throwable#addSuppressed
+     * suppressed}.
+     * <p>
+     * More generally, if it would cause problems for an
+     * exception to be suppressed, the {@code AutoCloseable.close}
+     * method should not throw it.
+     *
+     * <p>Note that unlike the {@link Closeable#close close}
+     * method of {@link Closeable}, this {@code close} method
+     * is <em>not</em> required to be idempotent.  In other words,
+     * calling this {@code close} method more than once may have some
+     * visible side effect, unlike {@code Closeable.close} which is
+     * required to have no effect if called more than once.
+     * <p>
+     * However, implementers of this interface are strongly encouraged
+     * to make their {@code close} methods idempotent.
+     */
+    @Override
+    public void close() throws Exception {
+        this.executorService.shutdownNow();
+    }
 }