Update actions/setup-python action to v6 (#928)

| datasource  | package              | from | to |
| ----------- | -------------------- | ---- | -- |
| github-tags | actions/setup-python | v5   | v6 |

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

Author: renovate[bot]

.github/workflows/system-testing.yml

@@ -32,7 +32,7 @@ jobs:
           echo "nginx_version=$(grep -m1 'FROM nginx:' <Dockerfile | awk -F'[: ]' '{print $3}')" >> $GITHUB_OUTPUT
 
       - name: Setup Python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version-file: ".python-version"
           cache: "pip"