Bump flat and gulp-mocha (#265)

Removes [flat](https://github.com/hughsk/flat). It's no longer used after updating ancestor dependency [gulp-mocha](https://github.com/sindresorhus/gulp-mocha). These dependencies need to be updated together.


Removes `flat`

Updates `gulp-mocha` from 7.0.2 to 8.0.0
- [Release notes](https://github.com/sindresorhus/gulp-mocha/releases)
- [Commits](sindresorhus/gulp-mocha@v7.0.2...v8.0.0)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: gulp-mocha
  dependency-type: direct:development
...

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