[SPARK-51009][INFRA] Add Daily Maven Java 21 CI for branch-4.0

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to add a daily Maven Java 21 CI for branch-4.0.

Why are the changes needed?

Like master branch, this PR will provide a test coverage for Apache Maven build and test capability additionally.

• https://github.com/apache/spark/actions/workflows/build_maven_java21.yml

Does this PR introduce any user-facing change?

No, this is a test infra change.

How was this patch tested?

Manual review.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Thank you, @HyukjinKwon . Merged to master.

[dongjoon-hyun]

For the record, I triggered the CI manually and verified that it checked out branch-4.0 and downloaded Java 21.

• https://github.com/apache/spark/actions/workflows/build_branch40_maven_java21.yml

...
/usr/bin/git log -1 --format=%H
68615c4275c678f7cc72f5caa0fa1eb8e39e89fb
...
Downloading Java 21.0.6+7 (Zulu) from https://cdn.azul.com/zulu/bin/zulu21.40.17-ca-jdk21.0.6-linux_x64.tar.gz ...