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 4.1.1 to 5.0.2
- [Release notes](https://github.com/hughsk/flat/releases)
- [Commits](https://github.com/hughsk/flat/compare/4.1.1...5.0.2)

Updates `gulp-mocha` from 7.0.2 to 8.0.0
- [Release notes](https://github.com/sindresorhus/gulp-mocha/releases)
- [Commits](https://github.com/sindresorhus/gulp-mocha/compare/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] <support@github.com>

package.json

@@ -21,7 +21,7 @@
     "convert-units": "^2.3.4",
     "gulp": "^4.0.2",
     "gulp-data": "^1.3.1",
-    "gulp-mocha": "^7.0.2",
+    "gulp-mocha": "^8.0.0",
     "gulp-mustache": "^4.1.2",
     "image-size": "^0.7.5",
     "imagemin-cli": "^7.0.0",