submodule: bump openthread from 91b7c3f to 4a9fb06 (#556)

Bumps [openthread](https://github.com/openthread/openthread) from `91b7c3f` to `4a9fb06`.
- [Commits](openthread/openthread@91b7c3f...4a9fb06)

---
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>