Revert "[ci.yaml] Migrate web engine to Mac 12 (#32337)" (flutter/engine#32340)

This reverts commit 40f02a1e7655a9323f92a78820ed558ef9bf6644.

engine/src/flutter/.ci.yaml

@@ -308,12 +308,19 @@ targets:
       caches: >-
         [
           {"name":"flutter_cocoapods","path":"cocoapods"},
+          {"name":"old_osx_sdk","path":"osx_sdk"},
+          {"name":"builder_mac_engine","path":"builder"},
           {"name":"openjdk","path":"java"}
         ]
       dependencies: >-
         [
           {"dependency": "goldctl"}
         ]
+      os: Mac-10.15
+      runtime_versions: >-
+        [
+        ]
+      xcode: 12c33 # xcode 12.3
     timeout: 60
     runIf:
       - DEPS