submodule: bump openthread from a9805ed to 6e46e2e (#766)

Bumps [openthread](https://github.com/openthread/openthread) from `a9805ed` to `6e46e2e`.
- [Commits](openthread/openthread@a9805ed...6e46e2e)

---
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>