Backport of "Add label to skip release note category validation"

What does this MR do and why?

Backports the commit adding the pipeline:skip-release-note-categories label to 19-1-stable-ee (originally merged to master as fbbe47cc89f7, "Add label to skip release note category validation").

Needed to unblock !244716 (merged), which already has this label applied but currently has no effect since this rule doesn't exist yet on 19-1-stable-ee.

The docs-lint release-note-categories job re-validates the entire doc/releases/ history against the current categories.yml on every change, and can fail on already-published, unrelated release notes whose category names have since been renamed or removed upstream. This adds a pipeline:skip-release-note-categories label that lets a merge request opt out of that job (when: never), without changing its default behavior for anyone else.

This is purely additive — it only adds a new rule branch alongside the job's existing trigger condition, and has no effect unless the label is explicitly applied.

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.