submodule: bump openthread from 0239041 to de8974a

Bumps [openthread](https://github.com/openthread/openthread) from `0239041` to `de8974a`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@0239041...de8974a)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: de8974aca4668e633c899297838162c0ea5def76
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]