Run microbenchmarks_ios only on ios/12 testbeds. (#50134)

This is to remove the flakiness associated with running those tests on
ios/13 when they always fail.

Context:
https://github.com/flutter/flutter/issues/49635

dev/devicelab/manifest.yaml

@@ -505,7 +505,9 @@ tasks:
     description: >
       Runs benchmarks from dev/benchmarks/microbenchmarks on iPhone 6.
     stage: devicelab_ios
-    required_agent_capabilities: ["mac/ios"]
+    #TODO(godofredoc): Remove ios/12 capability once
+    # https://github.com/flutter/flutter/issues/49635 is fixed.
+    required_agent_capabilities: ["mac/ios", "ios/12"]
     timeout_in_minutes: 30
 
   flutter_view_ios__start_up: