[keyserver] Remove experimental flags for Node.js

Summary:
Before these diff we had three experimental flags enabled:

1. --loader - we're keeping this one, which we use to make sure that when we import files in other folders of this monorepo (eg. lib, web, landing), we pass the Babel-transpiled version to Node – otherwise Node gets confused by the Flow annotations
2. --experimental-json-modules - removing this one, which should have been cut after we addressed ENG-929, which required removing all imports of JSON files in keyserver
3. --experimental-specifier-resolution=node - removing this one, as it is going after in Node 19 and doesn't work with Webpack 5. The earlier diffs in this stack updated all imports to work without this flag

Depends on D6694

Test Plan: I ran web, landing, keyserver, and native all together and made sure everything still worked

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6695