Update actions/checkout action to v4 (#5409)

Signed-off-by: milkmaker <milkmaker@mailcow.de>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/update_postscreen_access_list.yml

@@ -15,7 +15,7 @@ jobs:
    runs-on: ubuntu-latest
    steps:
     - name: Checkout
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     - name: Generate postscreen_access.cidr
       run: |