submodule: bump openthread from 21ac9f5 to 93e72bc

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

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

Signed-off-by: dependabot[bot] <[email protected]>