submodule: bump openthread from d7810fa to d60ec88 (#892)

Bumps [openthread](https://github.com/openthread/openthread) from `d7810fa` to `d60ec88`.
- [Commits](openthread/openthread@d7810fa...d60ec88)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>