[keyserver][lib] Simplify leaveThread/deleteThread 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.

Doing these two together because they share the same types and validators.

Depends on D8507

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/D8508