Only run tests on macOS 12 (flutter#5369)

There's an flutter/flutter#138238 with mac_toolchain that makes Xcode installs flakey and an flutter/flutter#138246 that makes Xcode installs more frequent on macOS 13, which is causing presubmit tests to fall frequently. In the meantime, we'll only have tests run on macOS 12.

.ci.yaml

@@ -85,7 +85,7 @@ platform_properties:
         [
           {"dependency": "ruby", "version": "ruby_3.1-pod_1.13"}
         ]
-      os: "Mac-12|Mac-13"
+      os: "Mac-12"
       device_type: none
       cpu: arm64
       # TODO(vashworth): Remove specific toolchain_ver once https://github.com/flutter/flutter/issues/138109 is resolved.
@@ -101,7 +101,7 @@ platform_properties:
         [
           {"dependency": "ruby", "version": "ruby_3.1-pod_1.13"}
         ]
-      os: "Mac-12|Mac-13"
+      os: "Mac-12"
       device_type: none
       cpu: x86
       # TODO(vashworth): Remove specific toolchain_ver once https://github.com/flutter/flutter/issues/138109 is resolved.