Set xcode version to older compatible version for microbenchmarks_ios_xcode_debug test (#134693)

Test added in flutter/flutter#134493 requires an
older version of Xcode because default version of Xcode is not
compatible with macOS version in CI.

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Author: vashworth

.ci.yaml

@@ -4088,6 +4088,10 @@ targets:
       tags: >
         ["devicelab", "ios", "mac"]
       task_name: microbenchmarks_ios_xcode_debug
+      $flutter/osx_sdk : >-
+        {
+          "sdk_version": "14c18"
+        }
     bringup: true
 
   - name: Mac_ios native_assets_ios_simulator