Promote java version in JavaToolchain

[slawekjaranowski]

Many plugins using toolchains has requirements to detect java version selected by toolchains in runtime.

Expose java version in JavaToolchain can simplify plugins code.

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[cstamas]

But the have at least two other Toolchains related PR somewhere in the queue. Can you check those out as well?

[slawekjaranowski]

I see:

• feat: Improve the matching of required versions #11770
• Feat: Improve matching of Toolchains versions #11786