submodule: bump openthread from d0fbfb8 to af18582 (#761)

Bumps [openthread](https://github.com/openthread/openthread) from `d0fbfb8` to `af18582`.
- [Commits](openthread/openthread@d0fbfb8...af18582)

---
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>