Bump flat and mocha in /challenges/planning

Bumps [flat](https://github.com/hughsk/flat) to 5.0.2 and updates ancestor dependency [mocha](https://github.com/mochajs/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 `mocha` from 6.2.0 to 10.2.0
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](mochajs/mocha@v6.2.0...v10.2.0)

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

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Author: dependabot[bot]