Bump third_party/openthread/repo from eaa6261 to 786bd7f (#29683)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `eaa6261` to `786bd7f`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@eaa6261...786bd7f)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]