Bump third_party/openthread/repo from d4155e4 to b73b124

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `d4155e4` to `b73b124`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@d4155e4...b73b124)

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

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