Reverts "Run iOS staging tests with Xcode 15.2" (#141412)

Reverts flutter/flutter#141392
Initiated by: QuncCccccc
This change reverts the following previous change:
Original Description:
Potential fix for https://github.com/flutter/flutter/issues/141383.

.ci.yaml

@@ -4185,7 +4185,7 @@ targets:
       # TODO(vashworth): Remove after Xcode 15 is default in CI (https://github.com/flutter/flutter/issues/132237)
       $flutter/osx_sdk : >-
         {
-          "sdk_version": "15c500b"
+          "sdk_version": "15a240d"
         }
       os: Mac-13
     bringup: true
@@ -4228,7 +4228,7 @@ targets:
       # TODO(vashworth): Remove after Xcode 15 is default in CI (https://github.com/flutter/flutter/issues/132237)
       $flutter/osx_sdk : >-
         {
-          "sdk_version": "15c500b"
+          "sdk_version": "15a240d"
         }
       os: Mac-13
     bringup: true
@@ -4338,7 +4338,7 @@ targets:
       # TODO(vashworth): Remove after Xcode 15 is default in CI (https://github.com/flutter/flutter/issues/132237)
       $flutter/osx_sdk : >-
         {
-          "sdk_version": "15c500b"
+          "sdk_version": "15a240d"
         }
       os: Mac-13
     bringup: true
@@ -4518,7 +4518,7 @@ targets:
       # TODO(vashworth): Remove after Xcode 15 is default in CI (https://github.com/flutter/flutter/issues/132237)
       $flutter/osx_sdk : >-
         {
-          "sdk_version": "15c500b"
+          "sdk_version": "15a240d"
         }
       os: Mac-13
     bringup: true