[web] Update to copy-webpack-plugin@11.0.0

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

Depends on D6706

Test Plan:

1. I tested dev build in web via yarn dev in web + keyserver and then tested the website
2. I tested prod build by running yarn prod in web and then tested the website by running keyserver with yarn prod-build && yarn-prod
3. I reviewed the releases on GitHub and though there were lots of breaking changes (mostly Node versions), found that none of them applied to us

Reviewers: atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6707