Update github actions/checkout from v2 to v4

https://github.com/actions/checkout/blob/main/CHANGELOG.md

.github/workflows/link-check.yml

@@ -37,7 +37,7 @@ jobs:
     if: ${{ github.repository == 'publiccodenet/url-check' }}
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: update-deps
         run: sudo apt-get update
       - name: upgrade-deps