submodule: bump openthread from 6943dd2 to 9099a13 (#578)

Bumps [openthread](https://github.com/openthread/openthread) from `6943dd2` to `9099a13`.
- [Commits](openthread/openthread@6943dd2...9099a13)

---
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>