[4.0] Rename `@apollo/client/link/core` to `@apollo/client/link`

[jerelmiller]

[jerelmiller]

Finished with #12591

[github-actions]

Do you have any feedback for the maintainers? Please tell us by taking a one-minute survey. Your responses will help us understand Apollo Client usage and allow us to serve you better.

[github-actions]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.
For general questions, we recommend using our Community Forum or Stack Overflow.