CI: move Ubuntu workflows to JDK 21

[Copilot]

NetBeans RELEASE300 dependencies are compiled for Java 21, while Ubuntu CI was still provisioning JDK 17. This updates Ubuntu-oriented GitHub Actions workflows to use JDK 21 so CI aligns with the project’s runtime baseline.

• Main CI workflow (maven.yml)

  • Updated both build and deploy jobs to provision JDK 21 via actions/setup-java@v4.

• Manual branch build workflow (build_branch.yml)

  • Updated branch build job to provision JDK 21 for Ubuntu-targeted ad hoc builds.

• Config normalization

  • Kept distribution/caching behavior unchanged; only Java version and step labels were adjusted.

- name: Set up JDK 21
  uses: actions/setup-java@v4
  with:
    java-version: '21'
    distribution: 'temurin'
    cache: maven

[Copilot]

Pull request overview

Updates the Apache NetBeans platform version used across the build by bumping the single netbeans.version Maven property from RELEASE290 to RELEASE300, thereby upgrading the module’s NetBeans dependencies from platform 29 → 30.

Changes:

• Bump netbeans.version property: RELEASE290 → RELEASE300
• Keep all NetBeans module dependency versions centralized via ${netbeans.version} (no additional version pinning introduced)

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[stefanofornari]

@jGauravGupta from the logs it looks like NB 30 dropped JDK17 and upgraded to JDK21. This is good time with our change as well. It should be enough to merge the new pipeline script in main to make this PR pass too.