Renames it to `ENABLED` to be consistent with other settings and
deprecates it.
I believe this change is necessary because other setting groups such as
`attachment`, `cors`, `mailer`, etc. have an `ENABLED` setting, but
`oauth2` is the only one with an `ENABLE` setting, which could cause
confusion for users.
This is no longer a breaking change because `ENABLE` has been set as
deprecated and as an alias to `ENABLED`.
