Add Gradle support for Java 21

What does this MR do?

This merge request adds Gradle 8.4, which is the version of Gradle that is compatible with Java 21.

What are the relevant issue numbers?

Dependency Scanning and License Scanning Java 2... (gitlab-org/gitlab#421172 - closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer