submodule: bump third_party/openthread/repo from a57d927 to `e7535f…

…7` (#2298)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `a57d927` to `e7535f7`.
- [Commits](openthread/openthread@a57d927...e7535f7)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

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