Update to Webpack 5

Summary:
I followed the official docs to update to Webpack 5. Details will be explained inline.

Depends on D6698

Test Plan:

1. I tested dev build in web via yarn dev in web + keyserver and then tested the website
2. I tested prod build by running yarn prod in web and then tested the website by running keyserver with yarn prod-build && yarn-prod
3. I tested native, just in case

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6699