submodule: bump openthread from 8c70c78 to 7cbfb8e

Bumps [openthread](https://github.com/openthread/openthread) from `8c70c78` to `7cbfb8e`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@8c70c78...7cbfb8e)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 7cbfb8edc9dbcd460490a4d09b048dd2c6aa8edd
  dependency-type: direct:production
...

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Author: dependabot[bot]