[lib] Update to mini-css-extract-plugin@2.7.2

Summary:
After updating to Webpack 5, I figured I'd update this related package as well.

Depends on D6704

Test Plan:

1. 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
2. I reviewed the releases on GitHub and saw only one release with breaking changes listed, none of which applied to us

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6705