UserAuth: clean-up auth entries on update

[kalleep]

What is this feature?
Because of how generic oauth has worked in the past, before #65902, we can have several copies of a user_auth entry for generic oauth. If we notice that we have updated more than one row we can remove all rows except one for the combination of user_id, auth_module and auth_id

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[gamab]

LGTM!