[keyserver] Update importJSON to use fs instead of import

Summary:
Replace the await import(../../${path}) syntax in importJSON to
use fs.readFileSync instead.

This addresses: https://linear.app/comm/issue/ENG-1182/update-importjson-to-use-fs-instead-of-import

Context: https://linear.app/comm/issue/ENG-929/fix-issues-with-node-1614-import-syntax-assertions

Test Plan:
The local instance of Comm remains running and does not crash
with these new changes. Also ran flow in the keyserver directory to
ensure that it was fine to remove the $FlowFixMe comment.

Reviewers: atul, ashoat, ginsu

Reviewed By: atul, ashoat

Subscribers: tomek, abosh

Differential Revision: https://phab.comm.dev/D5267