Run tests on JDK 21 instead of 17 #838
Description
Is your feature request related to a problem? Please describe.
Currently, all tests run on GraalVM 17 (and, experimentally, latest-ea JDK 25). As we intend to test features not present JDK 17 (but present on 21), we should bump the tests to use JDK21.
Describe the solution you'd like
We should change up the prepare-environment action to build the JAVA_HOME and GRAALVM_HOME separately. The JAVA_HOME should still remain on JDK 17, so we stay compatible with our current gradle wrapper, while the main tests should run on JDK 21 (the GRAALVM_HOME).
Describe alternatives you've considered
The only alternative is to not test new features, but this seems out of the question, and is not a viable alternative.
Additional context
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