[lib] Update terser-webpack-plugin to 4.2.3

Summary:

1. I moved this to lib... it's used in lib/webpack/shared.cjs, so I think that's where it should be
2. I updated to the latest version in the 4.x major version (1.6.2). I opted not to upgrade further since version 5 only works with Webpack 5

Depends on D6680

Test Plan:

1. I checked the breaking changes in the GitHub Releases changelog. I found this, which didn't list anything relevant
2. I checked the breaking changes in the CHANGELOG.md file. I found this and this, neither of which seemed relevant
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/D6681