submodule: bump openthread from 89b54dc to 473fbca (openthread#897)

Bumps [openthread](https://github.com/openthread/openthread) from `89b54dc` to `473fbca`.
- [Commits](openthread/openthread@89b54dc...473fbca)

---
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>