submodule: bump openthread from b3c4a7e to 7a2e337 (#928)

Bumps [openthread](https://github.com/openthread/openthread) from `b3c4a7e` to `7a2e337`.
- [Commits](openthread/openthread@b3c4a7e...7a2e337)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 7a2e337493475ff0cb679c240962691e010ecb30
  dependency-type: direct:production
...

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Author: dependabot[bot]