Remove deprecated features and enable future flag (#26066)
- Enables excludeDefaultMomentLocales by default
- Adds distDir cleaning (See RFC #6009)
- Adds support for `PORT`
- Removes `router.events` from the server-side router as it should not be used server-side (long-standing todo that is potentially breaking). Note that it's still available as `Router.events` (import Router from 'next/router') and with `useRouter` in `useEffect`. Using it with `useEffect` is the correct way and I've updated the upgrading guide to reflect that
- Added webpack 5 to the upgrading guide
- Removed `Head.rewind` as it's been a no-op since Next.js 9.5 and can now be safely removed from user code
Fixes #11408
Fixes #10338
Fixes #5554
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
## Feature
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
## Documentation / Examples
- [ ] Make sure the linting passes
