select-jdk-toolchain should be made threadSafe

### New feature, improvement proposal

I'm not sure about the semantics of the threadSafe marker (is there a clear definition of what it implies somewhere?),
but I think it would be nice if SelectJdkToolchainMojo could be thread-safe,
so that we don't receive related warning messages when using it in multithreaded builds.

Hopefully it is already thread-safe, but the marker was forgotten?