submodule: bump openthread from `512d072` to `a51a631` (#217)
submodule: bump openthread from `512d072` to `a51a631` (#217)

Bumps [openthread](https://github.com/openthread/openthread) from `512d072` to `a51a631`.
- [Commits](https://github.com/openthread/openthread/compare/512d0728895641061d25dbbc545f0256d3535865...a51a6311e96ab282d26f6700eb7c7448961596ce)

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

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-b91,openthread/ot-b91