[keyserver] Get rid of versions table and related endpoints

Summary:
We're not using this table and don't have any immediate plans to do so. If we do need to get the data versions was designed to store, we can easily get itfrom GitHub/AppStoreConnect/PlayStore APIs.

Context:
https://linear.app/comm/issue/ENG-3633/compute-equivalent-of-versions-table-using-ghappstoreplaystore-apis

https://linear.app/comm/issue/ENG-2428/compute-equivalent-of-keyserver-versions-table-with

Test Plan: Ran migration and my versions table is gone:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7439