ci: optimize GHA caching in ci.yaml using prime-maven-cache composite action

[lqiu96]

[DRAFT] - No need to review for now

Just an idea inspired from the zizmor workflow to have pre-reqs. Re-use the cache to try and speed up the setup time.

Part of #13296

[gemini-code-assist]

Code Review

This pull request introduces a new GitHub Actions composite action to prime the Maven cache by downloading dependencies during a pre-population step. The review feedback suggests updating the Java version from 25 to 21, as Java 25 is not yet a released LTS version, and refining the documentation to clarify that 'mvn install' is used for building snapshot artifacts to the local repository.

[gemini-code-assist]

There are two improvements needed here:

1. Java Version: The latest LTS version of Java is currently 21. Java 25 is not yet released as a General Availability (GA) LTS version, so specifying 25 may cause setup failures or use unstable early-access builds.
2. Documentation Clarification: In accordance with the project's general rules, we should clarify in the documentation that mvn install is used for building snapshot artifacts to the local ~/.m2 repository.

    # We default to the latest LTS version (21) to run the pre-population step.
    # Furthermore, GitHub Actions Cache entries are immutable by primary key: downstream jobs
    # restoring this cache can run 'mvn install' (which is used for building snapshot artifacts
    # to the local ~/.m2 repository) or add SNAPSHOT JARs locally without ever
    # polluting or overwriting the remote cache.
    - uses: actions/setup-java@c1e323688fd81a25caa38c78aa6df2d33d3e20d9 # v4
      with:
        java-version: 21
        distribution: 'temurin'
        cache: 'maven'

References

1. In documentation about the Java build process, clarify that mvn install is used for building snapshot artifacts to the local ~/.m2 repository.