Backport to 19.0: Release note CI job fixes: Skip label + MR pipelines only

What does this MR do and why?

Backports two CI/CD rule changes for .docs:rules:release-note-categories to 19-0-stable-ee. Only .gitlab/ci/rules.gitlab-ci.yml is touched.

1. Skip label (from !244752 (merged)): adds a pipeline:skip-release-note-categories label. When present on a merge request, the docs-lint release-note-categories job runs when: never, so you can edit an older release note whose category has since been renamed or removed from categories.yml without the job failing.
2. Run on merge requests (from !242541 (merged)): changes the job rule from if-default-refs to if-merge-request so validation runs on merge requests (only validating the release notes changed in the MR). This was missed when !244949 (merged) backported the skip label to the stable branches, and is required for the skip label to take effect — the label only applies in a merge-request context.

The documentation change from !244752 (merged) is omitted because doc/development/documentation/release_notes.md diverges significantly on this stable branch. The pipeline:skip-release-note-categories label already exists project-wide.

Related links

• Related to Add label to skip release note category validation (!244752 - merged)
• Related to Scope release-note category validation to chang... (!242541 - merged)
• Related to Backport of "Add label to skip release note cat... (!244949 - merged)
• Related to Release notes categories validation failure on ... (#607413)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.