Bump elliptic, @microsoft/sp-build-web, @microsoft/sp-webpart-workben…

…ch and webpack

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


Removes `elliptic`

Updates `@microsoft/sp-build-web` from 1.18.2 to 1.20.1

Updates `@microsoft/sp-webpart-workbench` from 1.11.0 to 1.12.1

Updates `webpack` from 4.42.0 to 5.93.0
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](webpack/webpack@v4.42.0...v5.93.0)

---
updated-dependencies:
- dependency-name: elliptic
  dependency-type: indirect
- dependency-name: "@microsoft/sp-build-web"
  dependency-type: direct:development
- dependency-name: "@microsoft/sp-webpart-workbench"
  dependency-type: direct:development
- dependency-name: webpack
  dependency-type: direct:development
...

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