ci: fix missing version number for actions/checkout

Author: cwillisf

.github/workflows/ghpages-cleanup.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out GH Pages branch
-        uses: actions/checkout
+        uses: actions/checkout@v4
         with:
           # replace `fetch-depth` with `shallow-since` if and when actions/checkout#619 (or an equivalent) gets merged
           fetch-depth: 0