submodule: bump openthread from 59b87d6 to c519aa4

Bumps [openthread](https://github.com/openthread/openthread) from `59b87d6` to `c519aa4`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@59b87d6...c519aa4)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>