[ci] Upgrade to Catalina (#49519)

Catalina VMs are using the latest major version of virtualization which has improved networking which should fix `Agent is not responding!` issue.

.cirrus.yml

@@ -442,7 +442,7 @@ task:
 # than just fetching the data in the first place.
 task:
   osx_instance:
-    image: mojave-flutter # see https://cirrus-ci.org/guide/macOS/ for list of images (we should update regularly)
+    image: catalina-flutter # see https://cirrus-ci.org/guide/macOS/ for list of images (we should update regularly)
     # cpu is always 2
     # memory is always 8G
   environment: