Check Markdown Links workflow does not meet GitHub actions requirements

The `check-markdown-links` workflow is failing during action setup before link checking runs.

Failure from PR #7175: https://github.com/dotnet/dotnet-docker/actions/runs/25524177932/job/74915653668

```text
The action reviewdog/action-setup@d8a7baabd7f3e8544ee4dbde3ee41d0011c3a93f is not allowed in dotnet/dotnet-docker because all actions must be from a repository owned by your enterprise, created by GitHub, or match one of the patterns: gaurav-nelson/github-action-markdown-link-check@v1, reviewdog/action-setup@v1, umbrelladocs/action-linkspector@v1.
```

The workflow uses `umbrelladocs/action-linkspector@v1`, which attempts to fetch `reviewdog/action-setup` by SHA. That does not satisfy the current allowed-actions policy, so the workflow cannot run successfully.