fix(ci): downgrade Python to 3.13 due to yamale incompatibility

Python 3.14 removed ast.Num/ast.Str/ast.Bytes deprecated classes,
causing yamale (used by chart-testing) to fail with AttributeError.

Downgrade to Python 3.13 until yamale is updated for 3.14 compatibility.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

.github/workflows/pr-checks.yaml

@@ -28,7 +28,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: '3.14'
+          python-version: '3.13'
 
       # Check 1: Verify helm-docs is up to date
       - name: Verify helm-docs up to date