Switch CI from macOS 11 (Big Sur) to 14 (Sonoma)

This commit switches the macOS CI job from macOS 11 Big Sur - which is
no longer available in GitHub Actions - to the latest macOS release
provided by GHActions, that being macOS 14 Sonoma.

Author: suve

.github/workflows/ci.yml

@@ -7,8 +7,8 @@ on:
     branches: [ master ]
 
 jobs:
-  macos-11-big-sur:
-    runs-on: macos-11
+  macos-14-sonoma:
+    runs-on: macos-14
     steps:
       - name: Install FPC
         run: |