submodule: bump openthread from 700f624 to 6f3b431 (#618)

Bumps [openthread](https://github.com/openthread/openthread) from `700f624` to `6f3b431`.
- [Commits](openthread/openthread@700f624...6f3b431)

---
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>