submodule: bump openthread from 3873c6f to 9cc1cd9 (#733)

Bumps [openthread](https://github.com/openthread/openthread) from `3873c6f` to `9cc1cd9`.
- [Commits](openthread/openthread@3873c6f...9cc1cd9)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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