Bump webpack-dev-server and @symfony/webpack-encore

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


Removes `webpack-dev-server`

Updates `@symfony/webpack-encore` from 4.7.0 to 5.1.0
- [Release notes](https://github.com/symfony/webpack-encore/releases)
- [Changelog](https://github.com/symfony/webpack-encore/blob/main/CHANGELOG.md)
- [Commits](symfony/webpack-encore@v4.7.0...v5.1.0)

---
updated-dependencies:
- dependency-name: webpack-dev-server
  dependency-version: 
  dependency-type: indirect
- dependency-name: "@symfony/webpack-encore"
  dependency-version: 5.1.0
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]