bump up to actions/setup-python v6

Author: WolfgangWaltenberger

.github/workflows/validation.yml

@@ -27,7 +27,7 @@ jobs:
     steps:
     - uses: actions/checkout@v5
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v5
+      uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies