submodule: bump openthread from 33574ad to 6cfe14c (#690)

Bumps [openthread](https://github.com/openthread/openthread) from `33574ad` to `6cfe14c`.
- [Commits](openthread/openthread@33574ad...6cfe14c)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]