Update cache action version to v6

Author: fabaff

.github/workflows/testing.yml

@@ -32,7 +32,7 @@ jobs:
 
     - name: Load cached venv
       id: cached-poetry-dependencies
-      uses: actions/cache@v4
+      uses: actions/cache@v6
       with:
         path: .venv
         key: venv-${{ runner.os }}-${{ hashFiles('**/poetry.lock') }}