submodule: bump openthread from 08a7600 to 88b2b5c

Bumps [openthread](https://github.com/openthread/openthread) from `08a7600` to `88b2b5c`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@08a7600...88b2b5c)

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

Signed-off-by: dependabot[bot] <support@github.com>