Address review comments of PR 40235

[michielbdejong]

Specifically nextcloud/server#40235 (comment) about using events, and two smaller issues.

[michielbdejong]

Need to think of a way to comply with https://github.com/nextcloud/server/pull/40235/files#r1390886920
Maybe have the MFA Checker app deal with this.
The more thorough option would be triggering additional pull requests in GSS, user_saml, and https://github.com/nextcloud/server/tree/master/lib/private/Authentication/TwoFactorAuth