* fix: Dovecot PassDB should restrict allowed auth mechanisms
This prevents PassDBs incompatible with certain auth mechanisms from logging failures which accidentally triggers Fail2Ban.
Instead only allow the PassDB to be authenticated against when it's compatible with the auth mechanism used.
* tests: Use `curl` for OAuth2 login test-cases instead of netcat
`curl` provides this capability for both IMAP and SMTP authentication with a bearer token. It supports both `XOAUTH2` and `OAUTHBEARER` mechanisms, as these updated test-cases demonstrate.
* chore: Add entry to `CHANGELOG.md`
