Bump webpack-dev-server and @microsoft/sp-build-web

Removes [webpack-dev-server](https://github.com/webpack/webpack-dev-server). It's no longer used after updating ancestor dependency [@microsoft/sp-build-web](http://aka.ms/spfx). These dependencies need to be updated together.


Removes `webpack-dev-server`

Updates `@microsoft/sp-build-web` from 1.16.1 to 1.21.1

---
updated-dependencies:
- dependency-name: webpack-dev-server
  dependency-version: 
  dependency-type: indirect
- dependency-name: "@microsoft/sp-build-web"
  dependency-version: 1.21.1
  dependency-type: direct:development
...

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Author: dependabot[bot]