Fix gradle plugin build error [gemnasium-maven only]

What does this MR do?

This MR fixes the configuration for the gradle plugin builder. It is using jcenter which was sunset some time ago: https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter/

This is the related job failure: https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium/-/jobs/3252722688

It's likely that jcenter is only down temporarily, but it's still worthwhile to change the repository url anyway.

Does this MR meet the acceptance criteria?

• (n/a) Changelog entry added
  • This only changes the build configuration and likely doesn't require a changelog entry.
• (n/a) Documentation created/updated for GitLab EE, if necessary
• (n/a) Documentation created/updated for this project, if necessary
• (n/a) Documentation reviewed by technical writer or follow-up review issue created
• (n/a) Tests added for this feature/bug
• (n/a) Job definition updated, if necessary
  • (n/a) Auto-DevOps template
  • (n/a) Job definition example
  • (n/a) CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer