[lib] Update to terser-webpack-plugin@5.3.6

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

Depends on D6705

Test Plan:

1. I tested dev build in landing and web via yarn dev in landing + web + keyserver and then tested the website
2. I tested prod build by running yarn prod in landing and web and then tested the website by running keyserver with yarn prod-build && yarn-prod
3. 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/D6706