chore(deps): bump merge, @microsoft/sp-build-web and @microsoft/sp-webpart-workbench

Removes [merge](https://github.com/yeikos/js.merge). It's no longer used after updating ancestor dependencies [merge](https://github.com/yeikos/js.merge), [@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 `merge`

Updates `@microsoft/sp-build-web` from 1.11.0 to 1.12.0

Updates `@microsoft/sp-webpart-workbench` from 1.11.0 to 1.12.0

---
updated-dependencies:
- dependency-name: merge
  dependency-version: 
  dependency-type: indirect
- dependency-name: "@microsoft/sp-build-web"
  dependency-version: 1.12.0
  dependency-type: direct:development
- dependency-name: "@microsoft/sp-webpart-workbench"
  dependency-version: 1.12.0
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]