Bump external/usersim from 951d1a1 to 6fe5f68

Bumps [external/usersim](https://github.com/microsoft/usersim) from `951d1a1` to `6fe5f68`.
- [Commits](microsoft/usersim@951d1a1...6fe5f68)

---
updated-dependencies:
- dependency-name: external/usersim
  dependency-type: direct:production
...

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