Bump third_party/openthread/repo from a8afffb to b78b71b (#30787)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `a8afffb` to `b78b71b`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@a8afffb...b78b71b)

---
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>