Bump third_party/openthread/repo from f0b6fce to 509596f

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `f0b6fce` to `509596f`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@f0b6fce...509596f)

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

Signed-off-by: dependabot[bot] <[email protected]>