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

Test added in https://github.com/flutter/flutter/pull/134493 requires an
older version of Xcode because default version of Xcode is not
compatible with macOS version in CI.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

.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