ci: Upgrade soon deprecated GitHub Actions (#43173)

I got so many warnings about [Node 20
EOL](https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/)
in the [try](https://github.com/servo/servo/actions/runs/22945103342):

> The following actions are running on Node.js 20 and may not work as
expected...

We avoid touching those actions forked from web-platform-tests.

Testing: If this can merge, it is successful.

---------

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

.github/actions/setup-python/action.yml

@@ -15,7 +15,7 @@ runs:
     # symlink to this version.
     - name: Setup system python
       if: ${{ inputs.skip-python-setup != 'true' }}
-      uses: actions/setup-python@v5
+      uses: actions/setup-python@v6
       with:
         python-version-file: ${{ inputs.python-version-file-path }}
     - name: Install uv