Allow specifying JVM toolchain version

[Jolanrensen]

Currently, the plugin produces different results based on the JVM toolchain version the project is built with.
This version, however, is often higher than the actual JVM target version of the library.

For instance, setting the JVM toolchain version to 21 produces

	public synthetic fun addFirst (Ljava/lang/Object;)V
	public fun addFirst (Ljava/lang/String;)V
	public synthetic fun addLast (Ljava/lang/Object;)V
	public fun addLast (Ljava/lang/String;)V

in the .api file for classes extending the List class, while these functions are not available in our JVM target version 11. (Correct me if I'm wrong btw)

It would be helpful if we could change the toolchain version for the api- tasks to avoid this and produce an api dump which actually represents our published artifact.

[fzhinkin]

@Jolanrensen, api-* tasks are only loading compiled class files (or klibs) and dump whatever declarations these files contain. So it's about how compilations are configured, not about a toolchain used for BCV-specific tasks.

[Jolanrensen]

Ah I see :). I actually found changing the jdk-release version on our end correctly changes the api dump. Just setting source- and target compatibility wasn't enough.