feat: switch from deprecated eslint-plugin-markdown to @eslint/markdown

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: remove or replace eslint-plugin-markdown #2273
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Swaps in-place from the old plugin to the new.

This isn't quite what the issue suggested, but I plan on sending a fast follow for the more full #1926 to migrate off of Markdownlint. I think it'll be nice to have a reference version of CTA that uses @eslint/markdown instead of eslint-plugin-markdown before the removal of Markdownlint. Just in case it's useful.

🎁

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.51%. Comparing base (92b8acf) to head (5edd463).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2297      +/-   ##
==========================================
- Coverage   98.61%   98.51%   -0.11%     
==========================================
  Files         139      139              
  Lines         941      941              
  Branches      243      243              
==========================================
- Hits          928      927       -1     
  Misses          9        9              
- Partials        4        5       +1     

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[github-actions]

🎉 This is included in version v2.53.0 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀