[ci] Update Xcode (#1049)

Updates to Xcode 13, which is now required by Flutter master. The use of 12.x is currently blocking the flutter->packages roller.
diff --git a/.cirrus.yml b/.cirrus.yml
index 48558fd..fd58d13 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -156,7 +156,7 @@
   name: local_tests
   use_compute_credits: $CIRRUS_USER_COLLABORATOR == 'true'
   osx_instance:
-    image: big-sur-xcode-12.4
+    image: big-sur-xcode-13
   env:
     PATH: $PATH:/usr/local/bin
     matrix: