Minor updates to the build files

[rock3r]

While I try to figure out what is happening with the IntelliJ Platform Gradle Plugin 2.0¹, I cleaned up the build scripts a bit. This should ensure that:

1. We use the same major JDK version for buildSrc too
2. We enforce the language version in the standalone sample too (it should come from the Jewel plugin, but it doesn't — possibly because we set a toolchain in the module, too)
3. There are a couple less minor formatting issues than before
4. We have one less unused function

Footnotes

1. For whatever reason, using any JDK 21 as Gradle JVM results in a classloading error at runtime when trying to use any class from the bridge module (but not its transitive dependencies). Even though the code is run with a JDK 21, both in bridge and standalone, Gradle needs to run with a JDK 22 for things to not blow up at runtime. ↩