[ci] Update macOS Cirrus image to Xcode 13 (flutter#4429)

In anticipation of flutter/flutter#91634 Flutter tool enforcement, and to use the latest Xcode and SDKs that our customers may be using, upgrade the macOS Cirrus image to Xcode 13.

Author: jmagman

.cirrus.yml

@@ -47,7 +47,7 @@ macos_template: &MACOS_TEMPLATE
   # PRs on macOS.
   use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true'
   osx_instance:
-    image: big-sur-xcode-12.5
+    image: big-sur-xcode-13
 
 # Light-workload Linux tasks.
 # These use default machines, with fewer CPUs, to reduce pressure on the
@@ -275,7 +275,7 @@ task:
         SIMCTL_CHILD_MAPS_API_KEY: ENCRYPTED[596a9f6bca436694625ac50851dc5da6b4d34cba8025f7db5bc9465142e8cd44e15f69e3507787753accebfc4910d550]
       create_simulator_script:
         - xcrun simctl list
-        - xcrun simctl create Flutter-iPhone com.apple.CoreSimulator.SimDeviceType.iPhone-11 com.apple.CoreSimulator.SimRuntime.iOS-14-5 | xargs xcrun simctl boot
+        - xcrun simctl create Flutter-iPhone com.apple.CoreSimulator.SimDeviceType.iPhone-11 com.apple.CoreSimulator.SimRuntime.iOS-15-0 | xargs xcrun simctl boot
       build_script:
         - ./script/tool_runner.sh build-examples --ios
       xcode_analyze_script: