submodule: bump openthread from de9f978 to 19dc5ce (#705)

Bumps [openthread](https://github.com/openthread/openthread) from `de9f978` to `19dc5ce`.
- [Commits](openthread/openthread@de9f978...19dc5ce)

---
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>