submodule: bump third_party/openthread/repo from 9106817 to 91b7c3f

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `9106817` to `91b7c3f`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@9106817...91b7c3f)

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

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