[keyserver][lib] Simplify createThread return type following deprecation of old codeVersions

Summary:
See the first diff in the stack. We're no longer expecting to see these old codeVersions, so we're simplifying this code.

Depends on D8506

Test Plan: I tested this whole stack together by playing around with the web app and making sure thread mutation endpoints seemed to work correctly

Reviewers: michal, kamil

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8507