[keyserver] Simplify various codepaths following deprecation of old codeVersions

Summary:
Grouped these together because they were all simple changes that didn't fit into any of the other diffs in the stack.

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 D8510

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/D8511