submodule: bump openthread from 21ac9f5 to 93e72bc (#575)

Bumps [openthread](https://github.com/openthread/openthread) from `21ac9f5` to `93e72bc`.
- [Commits](openthread/openthread@21ac9f5...93e72bc)

---
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>