Provide better support for toolchains for JVM projects

[KengoTODA]

Gradle 6.7+ provides the Toolchains for JVM projects, which makes it possible to specify which Java we use to compile the project. refs #187

Currently gradle-modules-plugin has two potential problems around this toolchain feature:

1. By default the toolchain feature adds the --release option via javaCompile.options.release property, and it conflicts with the --release option set by this plugin. Fixed by the commit 6163723

* What went wrong:
Execution failed for task :greeter.toolchain:compileJava.
> Cannot specify --release via `CompileOptions.compilerArgs` when using `JavaCompile.release`.

2. When a toolchain is enabled, the version of java compiler will be not the version of JVM running Gradle, but the version of JVM set via toolchain. So we should refer properties of the JavaToolchainSpec instead of JavaVersion.current(). Fixed by the commit 68ed16f

* What went wrong:
A problem occurred configuring project ':greeter.toolchain'.
> sourceCompatibility should not be set together with --release option

Based on the "Testing locally published plugin" guideline, I've confirmed that problems reproduced by the newly added test project have been resolved by these two commits.

Thanks for checking my PR! 🙌

[siordache]

Thanks for this PR!