Upgrade Python test --macos_minimum_os to 10.15 (Catalina)

See prior art:
- aa3b976
- 67fca5c

OSS foundational support matrix suggests that the minimum supported macOS
version has been moved up to macOS 11 (Big Sur).
See: https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

#test-continuous

PiperOrigin-RevId: 687413401

.github/workflows/test_python.yml

@@ -115,5 +115,5 @@ jobs:
           bazel: >-
             test ${{ matrix.targets }} ${{ matrix.flags }}
             --test_env=KOKORO_PYTHON_VERSION=${{ matrix.version }}
-            --macos_minimum_os=10.9
+            --macos_minimum_os=10.15
           exclude-targets: -//python/pb_unit_tests/...