Update to using the latest default macOS and Xcode toolchain

Author: tonyarnold

.github/workflows/continuous-integration.yml

@@ -1,7 +1,7 @@
 name: Continuous Integration
 
 env:
-  DEVELOPER_DIR: "/Applications/Xcode_12.5.app/Contents/Developer"
+  DEVELOPER_DIR: "/Applications/Xcode_13.2.1.app/Contents/Developer"
 
 on:
   pull_request: