[lib][landing][web] Update to css-loader@6.7.3
Summary:
After updating to Webpack 5, I figured I'd update this related package as well.
Depends on D6707
Test Plan:
- I tested dev build in web via yarn dev in web + keyserver and then tested the website
- I tested prod build by running yarn prod in web and then tested the website by running keyserver with yarn prod-build && yarn-prod
- I reviewed the releases on GitHub and found two versions with breaking changes listed: 5.0.0 and 6.0.0. Neither seemed to mentioned anything relevant to us
Reviewers: atul, tomek
Reviewed By: tomek
Differential Revision: https://phab.comm.dev/D6708