Bump lodash, @microsoft/sp-build-web, @microsoft/sp-webpart-workbench…

… and gulp

Bumps [lodash](https://github.com/lodash/lodash) to 4.17.21 and updates ancestor dependencies [lodash](https://github.com/lodash/lodash), [@microsoft/sp-build-web](http://aka.ms/spfx), [@microsoft/sp-webpart-workbench](http://aka.ms/spfx) and [gulp](https://github.com/gulpjs/gulp). These dependencies need to be updated together.


Updates `lodash` from 4.17.20 to 4.17.21
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](lodash/lodash@4.17.20...4.17.21)

Updates `@microsoft/sp-build-web` from 1.11.0 to 1.18.0

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

Updates `gulp` from 3.9.1 to 4.0.2
- [Release notes](https://github.com/gulpjs/gulp/releases)
- [Changelog](https://github.com/gulpjs/gulp/blob/master/CHANGELOG.md)
- [Commits](gulpjs/gulp@v3.9.1...v4.0.2)

---
updated-dependencies:
- dependency-name: lodash
  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: gulp
  dependency-type: direct:development
...

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