Deprecate /user/current/update

[ocket 8888]

There's a handler in Perl for `POST /user/current/update` which does
exactly what it looks like - updates the current user.
This can already be done by just doing `PUT /user/current` or ever simply
`PUT /users/{{ID}}`.
So can we deprecate this endpoint? I'll add a deprecation notice if we can
get a consensus on that.