Bump third_party/openthread/repo from b200c96 to 9b99fbf (#27196)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `b200c96` to `9b99fbf`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@b200c96...9b99fbf)

---
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>