submodule: bump openthread from 70b6c53 to ffe2f52 (#661)

Bumps [openthread](https://github.com/openthread/openthread) from `70b6c53` to `ffe2f52`.
- [Commits](openthread/openthread@70b6c53...ffe2f52)

---
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>