[lib] Replace optimize-css-assets-webpack-plugin with css-minimizer-webpack-plugin

Summary:
See the README for optimize-css-assets-webpack-plugin:

For webpack v5 or above please use css-minimizer-webpack-plugin instead.

This gets rid of a warning that was previously printing when generating a production Webpack build:

[DEP_WEBPACK_COMPILATION_OPTIMIZE_CHUNK_ASSETS] DeprecationWarning: optimizeChunkAssets is deprecated (use Compilation.hooks.processAssets instead and use one of Compilation.PROCESS_ASSETS_STAGE_* as stage option)

Depends on D6700

Test Plan:

1. Confirm the depercation warning goes away
2. I ran yarn prod in landing and web (we only use this plugin in prod builds) and then tested the website by running keyserver with yarn prod-build && yarn-prod

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6701