Target JVM 8 instead of JVM 11 for wider support

[shorinami]

As there is actually no expense involved from developers to support JVM 8 in Kotlin, it sounds reasonable to do so, as that is still a supported and widely used java version.

The only change necessary in this case is to change the jvmTarget kotlin option.

We should not change the java toolchain version, as some test dependencies require java 11 or higher. It does not affect the target version against which we compile anyway. For same reason there is no need to touch the git actions configuration.

[sksamuel]

Try the latest auto generated snapshot once it completes and if good I'll do a proper release.

[shorinami]

Try the latest auto generated snapshot once it completes and if good I'll do a proper release.

It does compile it for Java 8, although Gradle is still marking it as for Java 11 for whatever reason, so we should explicitly tell gradle what it is for:

   tasks.compileJava {
      targetCompatibility = "1.8"
      sourceCompatibility = "1.8"
   }

That fixes the problem, tested with local maven repository