Update GitHub Actions to use new QuantEcon action locations

- Replace QuantEcon/meta/.github/actions/link-checker@main
  with QuantEcon/action-link-checker@main in linkcheck.yml
- This migration moves from the old meta repository structure
  to the new dedicated action repositories

Author: mmcky

.github/workflows/linkcheck.yml

@@ -17,7 +17,7 @@ jobs:
         with:
           ref: gh-pages
       - name: AI-Powered Link Checker
-        uses: QuantEcon/meta/.github/actions/link-checker@main
+        uses: QuantEcon/action-link-checker@main
         with:
           html-path: '.'            # gh-pages live html
           fail-on-broken: 'false'