submodule: bump openthread from 8c2abba to c6bece0 (#367)

Bumps [openthread](https://github.com/openthread/openthread) from `8c2abba` to `c6bece0`.
- [Commits](openthread/openthread@8c2abba...c6bece0)

---
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>