fix: use consistent JDK version for CI and POM

[c-dilks]

• reverts PR Use JRE 25 in CI jobs #854, since we should test with the release specified in the POM file, and what is currently in the latest versioned clas12 environment module. If we test with a later version, we risk a dependency requiring a later JDK, and it won't be noticed by the CI.
• JDK 25 will still be tested, but restricted to unit tests only
• introduces string JAVA_VERSION, to indicate all the places we need to update when we want to increase the Java version

[baltzell]

let's do separate jobs for 21 and 25

[c-dilks]

let's do separate jobs for 21 and 25

That could be done in another PR; this PR is meant as a fix. Some approaches:

• expand the matrix jobs
• use a reusable workflow, with the Java version as an input variable
• manual workflow, with the Java version as an input variable
• create a draft PR proposing Java 25 in the CI, for occasional manual testing by rebasing

[c-dilks]

The unit test job is now a job matrix, for testing JDK versions JAVA_VERSION=21 and JAVA_VERSION_LATEST=25.