Docs: Remove 19.1-only categories HTML comment from release note template

What does this MR change and why?

The <!-- categories: ... --> comment was a 19.1-only step that duplicated the categories: frontmatter field. Now that 19.1 has shipped, this removes it in favour of the frontmatter field.

Removed from three places in doc/development/documentation/release_notes.md:

• The [!note] callout above the Feature release note template
• The trailing 19.1 sentence in the categories metadata-table row
• The <!-- categories: ... --> line in the example template body

This aligns with the release-notes-writer agent, which already dropped the 19.1 guidance.

Validation script

Release post category validation: https://gitlab.com/gitlab-org/gitlab/-/blob/master/scripts/lint/validate_release_categories.rb?plain=1

🔗 Related MRs

• !234830 (merged) "Adding release post category validation" — the original validator (Python). It only checked the HTML comment. That's where the comment requirement comes from.
• !239864 (merged) "Change categories validation script to use Ruby instead of Python" — includes frontmatter parsing and the "After release 19.1 → frontmatter" rule
• !239103 (merged) "Update RN process - Batch 1" — migrated the doc to the categories: frontmatter field
• !239148 (merged) "Update RN process - Batch 2" — removed the comment-based procedure steps