ci: Re-introduce "cirrus: switch to big-sur image"

Now that dlang/dmd#13279 is merged we can finally switch to macOS BigSur
on Cirrus CI on `master`.

This reverts commit 2985abd.

Author: PetarKirov

.cirrus.yml

@@ -58,9 +58,9 @@ task:
 
 # Mac
 task:
-  name: macOS 10.15 x64
+  name: macOS 11.x x64
   osx_instance:
-    image: catalina-xcode
+    image: big-sur-xcode
   timeout_in: 60m
   environment:
     OS_NAME: darwin