submodule: bump openthread from 97a3f76 to d60ec88

Bumps [openthread](https://github.com/openthread/openthread) from `97a3f76` to `d60ec88`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@97a3f76...d60ec88)

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

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