submodule: bump openthread from e7535f7 to 0abc8d7 (#532)

Bumps [openthread](https://github.com/openthread/openthread) from `e7535f7` to `0abc8d7`.
- [Commits](openthread/openthread@e7535f7...0abc8d7)

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

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