build(deps): bump flat and @vue/cli-plugin-unit-mocha in /frontend

Bumps [flat](https://github.com/hughsk/flat) to 5.0.2 and updates ancestor dependency [@vue/cli-plugin-unit-mocha](https://github.com/vuejs/vue-cli/tree/HEAD/packages/@vue/cli-plugin-unit-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](hughsk/flat@4.1.1...5.0.2)

Updates `@vue/cli-plugin-unit-mocha` from 4.5.13 to 5.0.8
- [Release notes](https://github.com/vuejs/vue-cli/releases)
- [Changelog](https://github.com/vuejs/vue-cli/blob/dev/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vue-cli/commits/v5.0.8/packages/@vue/cli-plugin-unit-mocha)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: "@vue/cli-plugin-unit-mocha"
  dependency-type: direct:development
...

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

Author: dependabot[bot]