submodule: bump openthread from 8f6d670 to fc30740

Bumps [openthread](https://github.com/openthread/openthread) from `8f6d670` to `fc30740`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@8f6d670...fc30740)

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

Signed-off-by: dependabot[bot] <[email protected]>