Revert workarounds for markdown-link-check issues

[creachadair]

#8339 works around some recurring markdown-link-check errors in CI with a temporary fork of the action package. Revert to the official package once those two issues are addressed.

1. fatal: unsafe repository (REPO is owned by someone else) with ubuntu 20.04 container actions/checkout#760. A git permissions issue,
   apparently triggered by a combination of a git change and the behaviour of
   actions/checkout.

2. Update markdown-lint-check to v3.10.2. gaurav-nelson/github-action-markdown-link-check#144.
   Merging an updated version of the underlying package that fixes a bug in the
   handling of local #anchors.

3. Once a newer version is tagged on the main repo (Request to tag a new release with anchor link fixes gaurav-nelson/github-action-markdown-link-check#147), switch the workflow configs back to that selfsame official (non-fork) repo.