Use maintained fork for markdown link check workflow

[matsumotosan]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did all existing and newly added tests pass locally?

What does this PR do?

Fixes #424

GitHub action for Markdown link checking has been using a deprecated repository. Switching to the maintained fork.

See warning on this CI run. Deprecated action uses very old versions of its dependencies.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

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📚 Documentation preview 📚: https://lit-utilities--445.org.readthedocs.build/en/445/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@3877a97). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff          @@
##             main   #445   +/-   ##
=====================================
  Coverage        ?    63%           
=====================================
  Files           ?     17           
  Lines           ?    740           
  Branches        ?      0           
=====================================
  Hits            ?    468           
  Misses          ?    272           
  Partials        ?      0           

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