submodule: bump openthread from 5493815 to a759a4a (#929)

Bumps [openthread](https://github.com/openthread/openthread) from `5493815` to `a759a4a`.
- [Commits](openthread/openthread@5493815...a759a4a)

---
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>