Moves the React hooks entrypoint from @apollo/client to @apollo/client/react

[MrDoomBringer]

See this milestone for more information: https://github.com/apollographql/apollo-client/issues/8190

This PR aims to remove the react entrypoint from @apollo/client and have it accessible only through @apollo/client/react.

• Adjusts @apollo/client and @apollo/client/react accordingly
• Adds an @apollo/testing/react entrypoint, used primarily for MockedProvider
• Includes a basic codemod to help move react hook imports out of /client and into client/react
• Includes a basic migration guide, currently only including information about this change
• Updates all AC React docs

Checklist:

• [X] If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• [X] Make sure all of the significant new logic is covered by tests

[hwillson]

Super excited about this! Let us know when you think this is ready for review @MrDoomBringer - thanks!

[jpvajda]

@MrDoomBringer 👋 can this be merged in now?

[MrDoomBringer]

@jpvajda Yup! Just waiting for an approval review now :-)