submodule: bump openthread from 64e0811 to 521f95f (#878)

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

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>