Fix refreshing of OAuth2 tokens in Sentry 9+

[stanhu]

Sentry has a background job that checks the state of all OAuth2 identities. If the OAuth2 refresh protocol doesn't work, it marks the identity as needing to reauthenticate.

The refresh protocol was failing because the client_id and client_secret were not set in https://github.com/getsentry/sentry/blob/8f717ad1198aa1549bf799b72efe880704f1c6fc/src/sentry/auth/providers/oauth2.py#L153-L154.