Move mac_android machines to mac_os 12.0.1. (#94771)

This is to keep all the mac machines on the same version. Mac machines
are being updated to Mac 12.0.1 to support iOS 15 phones as they are
getting forced updated.

Bug: https://github.com/flutter/flutter/issues/93667

.ci.yaml

@@ -85,7 +85,7 @@ platform_properties:
           {"dependency": "chrome_and_driver", "version": "version:84"},
           {"dependency": "open_jdk"}
         ]
-      os: Mac-10.15
+      os: Mac-12.0
       device_os: N
   mac_ios:
     properties: