submodule: bump third_party/openthread/repo from b212a0a to `91acbd…

…d` (#2191)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `b212a0a` to `91acbdd`.
- [Commits](openthread/openthread@b212a0a...91acbdd)

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

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