submodule: bump openthread from `4ff5ab6` to `df8e06a` (#58)
submodule: bump openthread from `4ff5ab6` to `df8e06a` (#58)

Bumps [openthread](https://github.com/openthread/openthread) from `4ff5ab6` to `df8e06a`.
- [Commits](https://github.com/openthread/openthread/compare/4ff5ab67d7cd6a6cbd28d55ed4e244fb9c3b8833...df8e06aaab507e070a9df682b6a28446a57122ef)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-nrf528xx,openthread/ot-nrf528xx