Update the mac_ios group to include newer dimension values for macOS,… (#94768)

* Update the mac_ios group to include newer dimension values for macOS, xocde and iOS.

This allows us to add a couple updated testbeds to the prod pool, and run jobs on them as well.
Once they are validated, we will move to update all testsbeds in prod and remove the older values
from the .ci.yaml file.

Bug:94767

* We are going to swap out the fleet in one shot since it doesn't seem to be possible to provide multi-values for these dimensions.

.ci.yaml

@@ -106,9 +106,9 @@ platform_properties:
           {"dependency": "gems"},
           {"dependency": "ios_signing"}
         ]
-      os: Mac-10.15
-      device_os: iOS-14.4.2
-      xcode: 12c33
+      os: Mac-12.0
+      device_os: iOS-15.1
+      xcode: 13a233
   mac_ios32:
     properties:
       caches: >-
@@ -248,7 +248,6 @@ targets:
 
   - name: Linux ci_yaml flutter roller
     recipe: infra/ci_yaml
-    bringup: true # TODO(chillers): https://github.com/flutter/flutter/issues/93225
     timeout: 30
     properties:
       tags: >