Bump third_party/openthread/repo from 809cdc4 to 8bc2504 (#27854)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `809cdc4` to `8bc2504`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@809cdc4...8bc2504)

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

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