CI: Downgrade to Python 3.10 for 'test-older-django' (#2955)

* CI: Downgrade Python used in 'test-older-django'

* Added a note to keep renovate.json python 'allowedVersions' in sync.

Author: intgr

.github/renovate.json

@@ -12,6 +12,10 @@
       {
           "matchPackageNames": ["mypy"],
           "rangeStrategy": "widen"
+      },
+      {
+          "matchPackageNames": ["python"],
+          "allowedVersions": "3.10"
       }
   ]
 }

.github/workflows/test.yml

@@ -97,7 +97,8 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v6
         with:
-          python-version: 3.14
+          # NOTE: Keep renovate.json python "allowedVersions" in sync
+          python-version: '3.10'
 
       - name: Install dependencies
         run: uv sync --no-dev --group tests