Bump engine.io, @microsoft/sp-build-web and @microsoft/sp-webpart-workbench

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


Removes `engine.io`

Updates `@microsoft/sp-build-web` from 1.4.1 to 1.15.2

Updates `@microsoft/sp-webpart-workbench` from 1.4.1 to 1.12.1

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

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Author: dependabot[bot]