Update build to MacOS 12 now that 11 is EOL for GitHub CI

We'd generally aimed to stick with older versions where possible (some
suggestions that it widens the built output compatibility?) but life
moves on and MacOS 11 is now thoroughly outdated anyway.

Author: pimterry

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
           - platform: windows
             os: "windows-2019"
           - platform: mac
-            os: "macos-11"
+            os: "macos-12"
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
@@ -32,7 +32,7 @@ jobs:
 
       # The API key in APPLE_API_KEY is a PEM cert that must be read from disk:
       - run: echo "$APPLE_API_KEY" > ./apple-api-key.p8
-        if: matrix.os == 'macos-11'
+        if: startsWith(matrix.os, 'macos-')
         env:
           APPLE_API_KEY: ${{ secrets.APPLE_API_KEY }}