[keyserver][lib] Restrict updateRoles endpoint to ThinThreadType

Summary:
Keyserver endpoints don't need to concern themselves with thick threads. I don't think we're using ThreadChanges for thick threads currently.

Depends on D13073

Test Plan: Flow, and reading through the code

Reviewers: tomek, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13074