Update flutter/engine to use Xcode 16, iOS 18 Simulator, and macOS 14

[vashworth]

Prerequisites

• Run all flutter/engine macOS tests using Xcode 16 and iOS 18 simulator #148906
• Upgrade 50% of bots in dart-internal to macOS 14.5 #148893 (assuming Xcode 16 requires macOS 14.5)
  • If Xcode 16 requires a higher macOS version, bots will need to be upgraded to new required version before doing
• Upgrade remaining chromium bots in each pool to macOS 14.5 #148892 (assuming Xcode 16 requires macOS 14.5)
  • If Xcode 16 requires a higher macOS version, bots will need to be upgraded to new required version before doing
• Ensure no open issues in ☂️ Tracking issue for any infra or test failures with Xcode 16 and iOS 18 Simulators #148898 under flutter/flutter

---

https://github.com/flutter/engine/blob/main/.ci.yaml

$flutter/osx_sdk : >-
        {
-          "sdk_version": "15a240d"
+          "sdk_version": "[insert new CIPD case-sensitive Xcode build version]"
        }
        
- os: Mac-13|Mac-14
+ os: Mac-14

https://github.com/flutter/engine/blob/main/ci/builders/local_engine.json
https://github.com/flutter/engine/blob/main/ci/builders/mac_android_aot_engine.json
https://github.com/flutter/engine/blob/main/ci/builders/mac_host_engine.json
https://github.com/flutter/engine/blob/main/ci/builders/mac_ios_engine.json
https://github.com/flutter/engine/blob/main/ci/builders/mac_unopt_debug_no_rbe.json

$flutter/osx_sdk : >-
        {
-          "sdk_version": "15a240d"
+          "sdk_version": "[insert new CIPD case-sensitive Xcode build version]"
        }
        
- os=Mac-13|Mac-14
+ os=Mac-14

https://github.com/flutter/engine/blob/main/ci/builders/mac_unopt.json

$flutter/osx_sdk : >-
        {
          "runtime_versions": [
-               "ios-17-0_15a240d"
+               "ios-18-0_[insert new CIPD case-sensitive Xcode build version]"
          ],
-          "sdk_version": "15a240d"
+          "sdk_version": "[insert new CIPD case-sensitive Xcode build version]"
        }
        
- os=Mac-13|Mac-14
+ os=Mac-14

https://github.com/flutter/engine/blob/main/testing/scenario_app/run_ios_tests.sh

- readonly OS_RUNTIME=com.apple.CoreSimulator.SimRuntime.iOS-17-0
- readonly OS="17.0"
+ readonly OS_RUNTIME=com.apple.CoreSimulator.SimRuntime.iOS-18-0
+ readonly OS="18.0"

In addition, golden files will need to be updated. See flutter/engine#49833 as example