chore(docs): update min. Xcode version to 12 (#1739)

Author: atierian

README.md

@@ -51,7 +51,7 @@ This library is licensed under the Apache 2.0 License.
 
 ## Installation
 
-Amplify requires Xcode 11.4 or higher to build.
+Amplify requires Xcode 12 or higher to build.
 
 | For more detailed instructions, follow the getting started guides in our [documentation site](https://docs.amplify.aws/lib/q/platform/ios)   |
 |-------------------------------------------------|