XEP-0480: Mechanism is upgraded on every log in but never offered

[mdosch]

Environment

• ejabberd version: 24.10.48-messaging.one
• Erlang version: unknown
• OS: unix/linux 6.1.0
• Installed from: unknown

Bug description

While the upgrade according to XEP-0480 works (upgrade from SCRAM-SHA-1 to SCRAM-SHA-512 via UPGR-SCRAM-SHA-512) SCRAM-SHA-512 is not offered during the next log in and the update is performed again.

So currently it just makes the authentication more expensive as it adds the overhead of upgrading to every log in but never offers the new mechanism. AFAIU ejabberd should offer SCRAM-SHA-512 to accounts which already performed the upgrade while keeping SCRAM-SHA-1 hashes stored as well for use with older clients. Probably it would make sense to store when a certain mechanism was last used and delete the corresponding hashes when not used in a configurable time frame.

CC: @tmolitor-stud-tu @weiss

[licaon-kter]

Duplicate of #4301 ?