Update remaining Python 3.8 upb macos tests to macos-13

macos-12 runner image is deprecated and is EOL 12/3/24 per actions/runner-images#10721

Note that Python 3.8 is EOL and these tests should also be dropped / upgraded in
favor of 3.9 soon anyways.

#test-continuous

PiperOrigin-RevId: 687002433

.github/workflows/test_upb.yml

@@ -186,11 +186,11 @@ jobs:
           # supported Python versions and assume this gives us sufficient test
           # coverage.
           - { os: ubuntu-latest, python-version: "3.8", architecture: x64, type: 'binary' }
-          - { os: macos-12, python-version: "3.8", architecture: x64, type: 'binary' }
+          - { os: macos-13, python-version: "3.8", architecture: x64, type: 'binary' }
           - { os: ubuntu-latest, python-version: "3.12", architecture: x64, type: 'binary' }
           - { os: macos-13, python-version: "3.12", architecture: x64, type: 'binary' }
           - { os: ubuntu-latest, python-version: "3.8", architecture: x64, type: 'source', continuous-only: true }
-          - { os: macos-12, python-version: "3.8", architecture: x64, type: 'source', continuous-only: true }
+          - { os: macos-13, python-version: "3.8", architecture: x64, type: 'source', continuous-only: true }
           - { os: ubuntu-latest, python-version: "3.12", architecture: x64, type: 'source', continuous-only: true }
           - { os: macos-13, python-version: "3.12", architecture: x64, type: 'source', continuous-only: true }