Replace deprecated URL checker action with maintained fork

gaurav-nelson/github-action-markdown-link-check is explicitly deprecated
and points to tcort/github-action-markdown-link-check as its successor.
The new action is actively maintained by the upstream markdown-link-check
package author and uses the same config file format.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tghosth

.github/workflows/url-checker.yml

@@ -30,7 +30,7 @@ jobs:
           fi
 
       - name: Check URLs
-        uses: gaurav-nelson/github-action-markdown-link-check@v1
+        uses: tcort/github-action-markdown-link-check@v1
         with:
           check-modified-files-only: ${{ steps.options.outputs.check-modified-files-only }}
           config-file: .github/workflows/config/url-checker-config.json