Use JVM Toolchain

[StefMa]

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description, Motivation and Context

This PR moves the JVM Version selection from the machine to the build time.
With that change, Gradle makes sure that the code will be compiled with the configured JVM ersion (21) from the configured vendor (Corretto/Amazon).
There is "no need" to install a specific JVM version on CI or your dev machine.
You only need "a" JVM installation that can start Gradle.
Gradle will detect that the configuration say it should compile the code with Java 21/Corretto, it will install the respective toolchain and use that to compile the code and run tests with that.

More information can also be found here:
https://docs.gradle.org/8.6/userguide/toolchains.html

🧪 How Has This Been Tested?

Remove your locally installed JVM version (21/Corretto).
Then use another JVM version and kick of Gradle (./gradlew check).
Wihin the configuration phase you will see an output like this:

Indicating that the requested toolchain (was donwloaded) and now installing on the machine for later usage.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[hartmut-co-uk]

Thanks, @StefMa, for submitting this PR!
I'll need to look closer at the gradle docs. But at first glance, this seems like a valid and useful addition.

[StefMa]

Hey @hartmut-co-uk, any chance to have a look at this?

[hartmut-co-uk]

Thanks @StefMa for picking this up again! I'm happy with this feature.

There's one detail that worries me - the volume of gradle dependencies cached jumps from 170MB to 356MB (due to the JDK...). So the data volumes written and read from cache is higher, also the JDK is downloaded with each and every workflow run.
But maybe that's generally a misconfiguration here?

Thank you very much for the contribution and apologies for the delay! 💚
Let's wrap this up quickly!

[hartmut-co-uk]

Suggested change

	languageVersion.set(JavaLanguageVersion.of(21))
	vendor.set(JvmVendorSpec.AMAZON)
	languageVersion = JavaLanguageVersion.of(21)
	vendor = JvmVendorSpec.AMAZON

Cosmetic only, but I think it would align better with the DSL used in this file in other places.