Bump actions/checkout to v4

Author: priteau

.github/workflows/stackhpc-pull-request.yml

@@ -77,7 +77,7 @@ jobs:
     if: github.repository == 'stackhpc/stackhpc-kayobe-config'
     steps:
       - name: GitHub Checkout 🛎
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Setup Python ${{ matrix.python-version }} 🐍
         uses: actions/setup-python@v4