[CI] Reduce to one macOS version

Due to the scarcity of GH provided runners, we'll only do one macOS version for now. The lead developers tend to run the latest macOS locally anyways, so it shouldn't be a problem to catch issues as they arise.

.github/workflows/main.yml

@@ -89,12 +89,11 @@ jobs:
 
   macos-multiple-pythons:
     name: macOS with Python ${{ matrix.python-version }}
-    runs-on: ${{ matrix.os }}
+    runs-on: macos-11
     timeout-minutes: 90
     strategy:
       matrix:
         python-version: [ '3.7', '3.9', '3.10' ]
-        os: ['macos-11', 'macos-12']
       fail-fast: false
     steps:
     # Attempt to fix intermittent cloning errors. The error message says something like