OAuth provider names are improperly documented

https://preprod.arkindex.teklia.com/api-docs/#operation/ListOAuthCredentials

The only valid provider name shown in the API docs is GitLabOAuthProvider, when the real name is just gitlab. This does not affect the database, so gitlab is truly stored and also used by the backend.

This also affects the Django admin: every existing OAuthCredentials instance is shown as not having any provider, just because gitlab is not in the list of valid choices.