update CI to macOS 15

Author: MattKiazyk

.github/workflows/ci.yml

@@ -2,10 +2,10 @@ name: CI
 on: [push, pull_request]
 jobs:
   test:
-    runs-on: macos-12
+    runs-on: macos-15
     steps:
     - uses: actions/checkout@v4
     - name: Run tests
       env: 
-        DEVELOPER_DIR: /Applications/Xcode_13.4.1.app
+        DEVELOPER_DIR: /Applications/Xcode_16.4.app
       run: swift test