[build] Add Java 21 and Java 25 to CI as advisory builds

Start with the existing Java 17 jobs in .github/workflows/maven.yml and .github/workflows/maven_on_pull_request.yml. Review the workflow matrix, Maven commands, fail-fast setting, and experimental error handling. Done means both workflows show builds for Java 17, 21, and 25 with only Java 17 gating as specified; branch protection still needs maintainer attention.

Motivation

XChange currently builds and tests only against Java 17 (<version.java>17</version.java> in pom.xml). As Java 21 and 25 see growing adoption, we want early visibility into compile/test compatibility on those JDKs without changing our supported target. The new matrix entries should be informational only — failures must not block PR merges or develop pushes.

Proposed change

Convert the single-JDK build in both workflows to a matrix over Java 17, 21, and 25. Java 17 remains the sole gating entry; 21 and 25 are marked experimental and use continue-on-error: ${{ matrix.experimental || false }} so they cannot fail the workflow.

Files:

• .github/workflows/maven.yml — push to develop. Java 17 entry keeps mvn clean deploy; 21/25 use mvn clean verify so no snapshots are published from non-17 JDKs.
• .github/workflows/maven_on_pull_request.yml — PRs to develop. All entries use mvn clean verify.

Both workflows set fail-fast: false so one experimental failure does not cancel the other JDK's run.

The Maven <release> target stays at 17 — we are testing that the project still builds/tests on newer JDKs, not retargeting bytecode.

Acceptance criteria

• Each workflow shows three matrix entries: build (17), build (21), build (25).
• A failure in build (21) or build (25) leaves the workflow's overall conclusion green.
• A failure in build (17) still turns the workflow red.
• On maven.yml, only the Java 17 entry runs deploy; 21 and 25 stop at verify.

Follow-up / caveats

• Branch protection. The required status check name changes from build to build (17). A maintainer with admin access to knowm/XChange needs to update the branch protection rule on develop accordingly, otherwise the required check will appear missing on new PRs.