submodule: bump third_party/openthread/repo from 48abf12 to `b200c9…

…6` (#1870)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `48abf12` to `b200c96`.
- [Commits](openthread/openthread@48abf12...b200c96)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

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