submodule: bump openthread from 07c56c2 to 21ac9f5 (#573)

Bumps [openthread](https://github.com/openthread/openthread) from `07c56c2` to `21ac9f5`.
- [Commits](openthread/openthread@07c56c2...21ac9f5)

---
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>