Support Java 17

[philipp94831]

java.lang.AbstractMethodError: Receiver class com.bakdata.util.seq2.Seq2Impl does not define or inherit an implementation of the resolved method 'java.util.List toList()' of interface com.bakdata.util.seq2.Seq2. is thrown in tests when using a gradle file containing

implementation(group = "com.bakdata.seq2", name = "seq2", version = "1.0.3")

configure<JavaPluginExtension> {
    sourceCompatibility = JavaVersion.VERSION_17
    targetCompatibility = JavaVersion.VERSION_17
}

However, it is fine to use it in the main sources