Update README.md

[laurenskling]

Remove references to 4.0 being a Release Candidate

Description of changes

Related issues (if any)

Testing

• [ ] List browser version(s) any admin UI changes were tested in:
• [ ] Please confirm you've added (or verified) test coverage for this change.
• [ ] Please confirm npm run test-all ran successfully.

[gautamsi]

You may want to mention the maintenance mode in this

[laurenskling]

I just ran into this. Writing about the maintenance mode in the main README sound like a good addon, next to this

[JedWatson]

@laurenskling given 4.0 has been out for so long now I removed the whole section