Bump third_party/openthread/repo from 837c7fd to 19af711

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `837c7fd` to `19af711`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@837c7fd...19af711)

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

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