submodule: bump third_party/openthread/repo from 75694d2 to a8afffb

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `75694d2` to `a8afffb`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@75694d2...a8afffb)

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

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