submodule: bump openthread from 91b7c3f to 1528c88 (#715)

Bumps [openthread](https://github.com/openthread/openthread) from `91b7c3f` to `1528c88`.
- [Commits](openthread/openthread@91b7c3f...1528c88)

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

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