[keyserver] Replace ashoat with admin

Summary: issue: https://linear.app/comm/issue/ENG-6614/configure-ashoatadmin-constant

Test Plan: Tested that creating a new db is successful - a user with the id provided by identity service is created and added to the users table(with the correct name), GENESIS is correctly created with creator id set to this id, user_id field is correctly filled

Reviewers: tomek, michal

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11105