[web][landing] Update webpack-dev-server and webpack-cli

Summary:
We use these libraries when running yarn dev / yarn prod in web / landing.

Depends on D6676

Test Plan:

1. Went through breaking changes for webpack-dev-server listed here
2. Went through changelog for webpack-cli here (no breaking changes listed)
3. Ran yarn dev and yarn prod in both web and landing. Ran the web app and landing page and played around

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6677