build(deps): bump flat and solidity-coverage in /contracts (#304)

Bumps [flat](https://github.com/hughsk/flat) and [solidity-coverage](https://github.com/sc-forks/solidity-coverage). These dependencies needed 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 `solidity-coverage` from 0.8.4 to 0.8.5
- [Release notes](https://github.com/sc-forks/solidity-coverage/releases)
- [Changelog](https://github.com/sc-forks/solidity-coverage/blob/master/CHANGELOG.md)
- [Commits](sc-forks/solidity-coverage@v0.8.4...v0.8.5)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: solidity-coverage
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>