Switching over from iOS-15 to iOS-16 in .ci.yaml. (#118807)

This will run tests on new iphone11 devicelab devices
which are running iOS-16.

Bug:flutter/flutter#117237

.ci.yaml

@@ -131,7 +131,7 @@ platform_properties:
         ]
       os: Mac-12
       cpu: x86
-      device_os: iOS-15
+      device_os: iOS-16
       xcode: 14c18 # Xcode 14.2 to support iOS 16.2 in devicelab.
   mac_arm64_ios:
     properties:
@@ -143,7 +143,7 @@ platform_properties:
         ]
       os: Mac-12
       cpu: arm64
-      device_os: iOS-15
+      device_os: iOS-16
       xcode: 14c18 # Xcode 14.2 to support iOS 16.2 in devicelab.
   windows:
     properties: