Bump python version to 3.9 for gcloud 460.0.0

PiperOrigin-RevId: 599953322

Author: mkruskal-google

.github/workflows/test_cpp.yml

@@ -367,7 +367,7 @@ jobs:
             vsversion: '2019'
             cache-prefix: windows-2019-cmake
             # windows-2019 has python3.7 installed, which is incompatible with the latest gcloud
-            python-version: '3.8'
+            python-version: '3.9'
           - name: Windows CMake 32-bit
             os: windows-2022
             flags: >-