Don't run Review App jobs for backstage changes

What does this MR do?

1. Rename the current .only-code-changes to .only:changes-code-backstage
2. Rename the current .only-code-qa-changes to .only:changes-code-backstage-qa
3. Create a new .only:changes-code (only code changes without backstage changes) and use it for package-and-qa-manual
4. Create a new .only:changes-code-qa (only code and QA changes without backstage changes) and use it for schedule:package-and-qa and the review-* jobs

The goal is to avoid spinning up a Review App if the MR only includes backstage changes.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and Testing

• Open a MR based and targeting this MR's branch and expect that no review-* jobs are created: !19102 (closed)
  • That works: "only" 154 jobs created instead of 174: https://gitlab.com/gitlab-org/gitlab/pipelines/91314273

Closes #30121 (closed).