[keyserver] Use verifyUserLoggedIn instead of fetchIdentityInfo in versionResponseValidator

Summary:
verifyUserLoggedIn returns the same thing as fetchIdentityInfo. The difference is that verifyUserLoggedIn will attempt identity login if the identity info is not present in MariaDB.

This makes it a bit more convenient to be able to eg. clear the identity info from MariaDB to force a re-login, which came up in ENG-6466.

Test Plan:

1. Flow
2. Careful reading of the code for verifyUserLoggedIn
3. An observation of what happened when I restarted my keyserver after clearing the identity info from MariaDB (restart triggered running verifyUserLoggedIn)

Reviewers: varun, inka

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10716