[keyserver] Use specs to compute invalid top-level keys

Summary:
Iterate over specs and check if their key is mentioned as invalid.

https://linear.app/comm/issue/ENG-4631/migrate-checkstate-from-session-utils-to-a-spec

Depends on D8948

Test Plan: Modify keyserver db and check if the store get fixed.

Reviewers: kamil, bartek, inka

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8949