Bump eventsource and webpack-dev-server in /client

Removes [eventsource](https://github.com/EventSource/eventsource). It's no longer used after updating ancestor dependency [webpack-dev-server](https://github.com/webpack/webpack-dev-server). These dependencies need to be updated together.


Removes `eventsource`

Updates `webpack-dev-server` from 2.9.7 to 4.11.1
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](webpack/webpack-dev-server@v2.9.7...v4.11.1)

---
updated-dependencies:
- dependency-name: eventsource
  dependency-type: indirect
- dependency-name: webpack-dev-server
  dependency-type: direct:development
...

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Author: dependabot[bot]