submodule: bump openthread from 9137b82 to 7048835 (#1278)

Bumps [openthread](https://github.com/openthread/openthread) from `9137b82` to `7048835`.
- [Commits](openthread/openthread@9137b82...7048835)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 7048835ba191faa82d3d8fe6c38999306c6a95fc
  dependency-type: direct:production
...

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Author: dependabot[bot]