Bump third_party/openthread/repo from b78b71b to ac7f751

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `b78b71b` to `ac7f751`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@b78b71b...ac7f751)

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

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