Bump flat and gulp-mocha

Bumps [flat](https://github.com/hughsk/flat) to 5.0.2 and updates ancestor dependency [gulp-mocha](https://github.com/sindresorhus/gulp-mocha). These dependencies need to be updated together.


Updates `flat` from 4.1.0 to 5.0.2
- [Release notes](https://github.com/hughsk/flat/releases)
- [Commits](hughsk/flat@4.1.0...5.0.2)

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
...

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

Author: dependabot[bot]