Bump third_party/openthread/repo from a0ff14c to afbb2d5

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `a0ff14c` to `afbb2d5`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@a0ff14c...afbb2d5)

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

Signed-off-by: dependabot[bot] <support@github.com>