Bump third_party/openthread/repo from cf357d7 to b212a0a (#32067)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `cf357d7` to `b212a0a`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@cf357d7...b212a0a)

---
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>