Bump external/usersim from e493ccd to 68b423a (#139)

Bumps [external/usersim](https://github.com/microsoft/usersim) from `e493ccd` to `68b423a`.
- [Commits](microsoft/usersim@e493ccd...68b423a)

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

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