Unpin mac_toolchain version (#5683)

mac_toolchain has been updated to no longer have 2 minute slowdown (flutter/flutter#138109), so unpin to use the latest version.

Author: vashworth

.ci.yaml

@@ -88,12 +88,9 @@ platform_properties:
       os: Mac-12|Mac-13
       device_type: none
       cpu: arm64
-      # TODO(vashworth): Remove specific toolchain_ver once https://github.com/flutter/flutter/issues/138109 is resolved.
       $flutter/osx_sdk : >-
         {
-          "sdk_version": "14e300c",
-          "toolchain_ver_arm": "JiMOaZvCH66lnGWMJdLScE-7lkUnKaw3COdWwqXmjWEC",
-          "toolchain_ver_intel": "Zp2HmSx7_-pu-yHqYqzoEpo0JRwNZGGwSj5V1SnLFmoC"
+          "sdk_version": "14e300c"
         }
   mac_x64:
     properties:
@@ -104,12 +101,9 @@ platform_properties:
       os: Mac-12|Mac-13
       device_type: none
       cpu: x86
-      # TODO(vashworth): Remove specific toolchain_ver once https://github.com/flutter/flutter/issues/138109 is resolved.
       $flutter/osx_sdk : >-
         {
-          "sdk_version": "14e300c",
-          "toolchain_ver_arm": "JiMOaZvCH66lnGWMJdLScE-7lkUnKaw3COdWwqXmjWEC",
-          "toolchain_ver_intel": "Zp2HmSx7_-pu-yHqYqzoEpo0JRwNZGGwSj5V1SnLFmoC"
+          "sdk_version": "14e300c"
         }
 
 targets: