[lib] Update optimize-css-assets-webpack-plugin to 6.0.1

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 from 5.0.3 to 6.0.1

Depends on D6679

Test Plan:

1. I tried to check release notes on GitHub, but the repo has neither a CHANGELOG file nor anything listed under GitHub releases. I also noticed it appears to be replaced with a different module for Webpack 5
2. I changed the 6.0.1 tag in the aforementioned repo, and it looks like the only thing that happens in 6.0 is that some dependencies are upgraded
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/D6680