Update next React version (#21647)

This does not mean that a release of 18.0 is imminent, only that the
main branch includes breaking changes.

Also updates the versioning scheme of the `@next` channel to include
the upcoming semver number, as well as the word "alpha" to indicate the
stability of the release.

- Before:       0.0.0-e0d9b28999
- After:        18.0.0-alpha-e0d9b28999