Bump third_party/openthread/repo from b212a0a to 8e9d58f

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `b212a0a` to `8e9d58f`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@b212a0a...8e9d58f)

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

Signed-off-by: dependabot[bot] <[email protected]>