chore: update actions/setup-python to v6

Signed-off-by: Jeff Mesnil <jmesnil@ibm.com>

Author: jmesnil

.github/workflows/run-tck.yml

@@ -51,7 +51,7 @@ jobs:
           path: a2a-tck
           ref: ${{ env.TCK_VERSION }}
       - name: Set up Python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version-file: "a2a-tck/pyproject.toml"
       - name: Install uv and Python dependencies