submodule: bump third_party/openthread/repo from 64e0811 to `521f95…

…f` (#2700)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `64e0811` to `521f95f`.
- [Commits](openthread/openthread@64e0811...521f95f)

---
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>