[keyserver][lib][native][web] Remove urlPrefix from BaseAppState

Summary: issue: https://linear.app/comm/issue/ENG-4459/refactor-urlprefix-field

Test Plan: ran yarn flow-all. Checked that the app still works - connects to the keyserver and its possible to send messages. Checked that the migration is succeful

Reviewers: michal, kamil

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8815