submodule: bump openthread from a1816c1 to b78b71b (#642)

Bumps [openthread](https://github.com/openthread/openthread) from `a1816c1` to `b78b71b`.
- [Commits](openthread/openthread@a1816c1...b78b71b)

---
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>