Update builder configurations to apply the new generators format. (flutter/engine#28038)

Web engine required more complex generator configurations which forced
us change the generator values from list to a dict.

Bug: https://github.com/flutter/flutter/issues/81855

engine/src/flutter/ci/builders/mac_ios_engine.json

@@ -47,12 +47,14 @@
             "mac_model=Macmini8,1",
             "os=Mac"
         ],
-        "generators": [
-            {
-                "name": "BuildObjcDoc",
-                "script": "flutter/tools/gen_objcdoc.sh"
-            }
-        ],
+        "generators": {
+		"tasks": [
+			{
+				"name": "BuildObjcDoc",
+				"scripts": ["flutter/tools/gen_objcdoc.sh"]
+			}
+		]
+	},
         "gn": [
             "--bitcode",
             "--ios",

engine/src/flutter/ci/builders/mac_ios_engine_profile.json

@@ -47,7 +47,7 @@
             "mac_model=Macmini8,1",
             "os=Mac"
         ],
-        "generators": [],
+        "generators": {},
         "gn": [
             "--ios",
             "--runtime-mode",

engine/src/flutter/ci/builders/mac_ios_engine_release.json

@@ -49,7 +49,7 @@
             "mac_model=Macmini8,1",
             "os=Mac"
         ],
-        "generators": [],
+        "generators": {},
         "gn": [
             "--ios",
             "--runtime-mode",

engine/src/flutter/ci/builders/windows_host_engine.json

@@ -67,7 +67,7 @@
         "gclient_custom_vars": {
           "download_android_deps": false
         },
-        "generators": [],
+        "generators": {},
         "gn": [
             "--runtime-mode",
             "release",