chore(lint): allow footnote definitions in ReferenceLinks Vale rule

Description

This MR updates the ReferenceLinks Vale style rule so it no longer flags Markdown footnote definitions as reference-style links. The old regular expression matched any [label]: pattern, which incorrectly caught footnote definitions like [^label]: Footnote text. alongside genuine reference-style links like [label]: https://example.com. The new pattern excludes labels that start with a caret, so footnotes pass while reference-style links are still flagged.

This brings the rule in step with the fix already merged in the gitlab project, which is the source of truth for Vale and markdownlint configuration.

Related issues and merge requests

• Upstream fix in the gitlab project (source of truth), merged
• Style guide update that depends on this fix, in review
• Tracking issue

Documentation

If this MR adds or updates CLI command documentation, follow the conventions in the GitLab CLI (glab) documentation style guide, run make gen-docs, and commit the updated files in docs/source/.