[keyserver] Use fetchServerThreadInfos instead of fetchThreadInfos for updateRole

Summary:
Using fetchThreadInfos in updateRole with the commbot ghost viewer results in the request's thread being filtered out. We instead use fetchServerThreadInfos instead here to avoid filtering based on the viewer

Depends on D13832

Test Plan: No longer received ServerError: invalid_parameters later in stack when attempting to create a farcaster channel community with farcaster bot and updating role with commbot as viewer

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13833