submodule: bump openthread from 2cb137f to b6f6d34 (#1275)

Bumps [openthread](https://github.com/openthread/openthread) from `2cb137f` to `b6f6d34`.
- [Commits](openthread/openthread@2cb137f...b6f6d34)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: b6f6d34606316f6511dfce2a496ea0953876b50a
  dependency-type: direct:production
...

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Author: dependabot[bot]