submodule: bump openthread from 971f05f to 3560870 (#859)

Bumps [openthread](https://github.com/openthread/openthread) from `971f05f` to `3560870`.
- [Commits](openthread/openthread@971f05f...3560870)

---
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>