Add deprecation for global installs

Created by: mrmckeb

Is your proposal related to a problem?

When rolling out new features that require changes to the create-react-app package, we regularly see issues related to users with older, global installs of that package.

Describe the solution you'd like

Ideally, we should add a deprecation warning in react-scripts so that when it a user attempts to install it with a globally installed create-react-app, they're asked to remove it and use npx or yarn create instead.

Describe alternatives you've considered

Alternatively, or additionally, we could add this to create-react-app itself, but that won't fix the issue for those with older installs.

Additional context

This came out of a discussion with @iansu.