Update Python version in mkdocs_publish workflow

Author: ceesem

.github/workflows/mkdocs_publish.yml

@@ -17,7 +17,7 @@ jobs:
           git config user.email 41898282+github-actions[bot]@users.noreply.github.com
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.x
+          python-version: 3.13
       - run: echo "cache_id=$(date --utc '+%V')" >> $GITHUB_ENV 
       - uses: actions/cache@v4
         with: