submodule: bump third_party/openthread/repo from 5197402 to d81c6fa (openthread#2155)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `5197402` to `d81c6fa`.
- [Commits](openthread/openthread@5197402...d81c6fa)

---
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>

Author: dependabot[bot]