submodule: bump openthread from 518a849 to be06d7c (#486)

Bumps [openthread](https://github.com/openthread/openthread) from `518a849` to `be06d7c`.
- [Commits](openthread/openthread@518a849...be06d7c)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>