submodule: bump openthread from 3bc82a9 to 6526606 (#1149)

Bumps [openthread](https://github.com/openthread/openthread) from `3bc82a9` to `6526606`.
- [Commits](openthread/openthread@3bc82a9...6526606)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 6526606739aa5c37c6589e862ff614621206d9ab
  dependency-type: direct:production
...

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Author: dependabot[bot]