Bump CI to macOS 12

Features Xcode 13.3.1, consistent with local development.

Maybe works around accent color bug.

.github/workflows/private_beta.yml

@@ -16,7 +16,7 @@ env:
 jobs:
   build_upload:
     name: Distribute Private Beta
-    runs-on: macos-11
+    runs-on: macos-12
     environment:
       name: private_beta
     strategy:

.github/workflows/release.yml

@@ -15,7 +15,7 @@ env:
 jobs:
   build_upload:
     name: Upload to ASC
-    runs-on: macos-11
+    runs-on: macos-12
     strategy:
       fail-fast: true
       matrix:

.github/workflows/test.yml

@@ -15,7 +15,7 @@ on:
 jobs:
   run_tests:
     name: Run tests
-    runs-on: macos-11
+    runs-on: macos-12
     timeout-minutes: 20
     steps:
       - uses: actions/checkout@v2