Run test with older version of Xcode (#137293)

Xcode Debug tests require an older version of Xcode to run in CI.

New test is failing due to incompatible Xcode version: https://ci.chromium.org/ui/p/flutter/builders/staging/Mac_arm64_ios%20flavors_test_ios_xcode_debug/1/overview

Author: vashworth

.ci.yaml

@@ -3934,6 +3934,10 @@ targets:
       tags: >
         ["devicelab", "ios", "mac"]
       task_name: flavors_test_ios_xcode_debug
+      $flutter/osx_sdk : >-
+        {
+          "sdk_version": "14c18"
+        }
     bringup: true
 
   - name: Mac_ios flutter_gallery_ios__compile