Upgraded GitHub Actions versions and switched to Java 21 in workflows.

[mike-neck]

Summary

• Bump versions of GitHub Actions used in the Build workflow to currently supported releases:
  • actions/checkout
  • actions/setup-java
  • actions/upload-artifact
• This addresses recent failures where the Build workflow could not run due to outdated action versions.
• Upgraded Java version to 21, which is recommended in document.

Motivation

• The target repository’s Build workflow has been failing because it relied on older action versions that are no longer supported on GitHub-hosted runners. Updating to maintained versions restores the CI pipeline.

What Changed

• Updated the above actions to their current, supported major versions.
• No changes to plugin sources or build logic; CI configuration only.