submodule: bump openthread from 7a2983b to b2c8147 (#455)

Bumps [openthread](https://github.com/openthread/openthread) from `7a2983b` to `b2c8147`.
- [Commits](openthread/openthread@7a2983b...b2c8147)

---
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>