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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`.