[keyserver] Update assets.json imports to use fs instead of import

Summary:
Change await import to fs.readFileSync when loading the
assets.

Test Plan:
On both landing and web, ran yarn prod, then `yarn
prod-build` followed by yarn prod in keyserver. Both pages load fine.
Also changed the Node version in .nvmrc to 18.11.0 and they run fine as
well.

Reviewers: atul, ginsu, abosh, ashoat

Reviewed By: atul, ginsu, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5371