submodule: bump openthread from 5bd9de5 to c519aa4 (#920)

Bumps [openthread](https://github.com/openthread/openthread) from `5bd9de5` to `c519aa4`.
- [Commits](openthread/openthread@5bd9de5...c519aa4)

---
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>