submodule: bump openthread from b0e7b14 to 956e1a0

Bumps [openthread](https://github.com/openthread/openthread) from `b0e7b14` to `956e1a0`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@b0e7b14...956e1a0)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 956e1a0c7d5a8c40d9f5c38c604fb8a00009600d
  dependency-type: direct:production
...

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Author: dependabot[bot]