Bump flat and hardhat in /contracts (#679)

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


Updates `flat` from 4.1.1 to 5.0.2
- [Commits](hughsk/flat@4.1.1...5.0.2)

Updates `hardhat` from 2.6.1 to 2.15.0
- [Release notes](https://github.com/nomiclabs/hardhat/releases)
- [Commits](https://github.com/nomiclabs/hardhat/compare/hardhat-core-v2.6.1...hardhat@2.15.0)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: hardhat
  dependency-type: direct:development
...

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