Upgrade macos-11 tests to macos-12

Github dropped macos-11 on their runners on June 28th.  This causes all our jobs to stall looking for a suitable runner.

PiperOrigin-RevId: 648527302

.github/workflows/test_upb.yml

diff --git a/.github/workflows/test_upb.yml b/.github/workflows/test_upb.yml
index e25fa60..27f909a 100644
--- a/.github/workflows/test_upb.yml
+++ b/.github/workflows/test_upb.yml
@@ -171,11 +171,11 @@
           - { os: ubuntu-latest, python-version: "3.8", architecture: x64, type: 'binary' }
           - { os: macos-11, python-version: "3.8", architecture: x64, type: 'binary' }
           - { os: ubuntu-latest, python-version: "3.11", architecture: x64, type: 'binary' }
-          - { os: macos-12, python-version: "3.11", architecture: x64, type: 'binary' }
+          - { os: macos-11, python-version: "3.11", architecture: x64, type: 'binary' }
           - { os: ubuntu-latest, python-version: "3.8", architecture: x64, type: 'source' }
           - { os: macos-11, python-version: "3.8", architecture: x64, type: 'source' }
           - { os: ubuntu-latest, python-version: "3.11", architecture: x64, type: 'source' }
-          - { os: macos-12, python-version: "3.11", architecture: x64, type: 'source' }
+          - { os: macos-11, python-version: "3.11", architecture: x64, type: 'source' }
 
           # Windows uses the full API up until Python 3.10, so each of these
           # jobs tests a distinct binary wheel.