submodule: bump openthread from 9f6921c to 944a246 (openthread#975)

Bumps [openthread](https://github.com/openthread/openthread) from `9f6921c` to `944a246`.
- [Commits](openthread/openthread@9f6921c...944a246)

---
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>