submodule: bump openthread from 4459c54 to bb5e9b7

Bumps [openthread](https://github.com/openthread/openthread) from `4459c54` to `bb5e9b7`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@4459c54...bb5e9b7)

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

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