submodule: bump openthread from `4e52ffb` to `73c4859` (#455)
submodule: bump openthread from `4e52ffb` to `73c4859` (#455)

Bumps [openthread](https://github.com/openthread/openthread) from `4e52ffb` to `73c4859`.
- [Commits](https://github.com/openthread/openthread/compare/4e52ffb54e4228f8aa975a76ebc8835cc752d7d5...73c48595b7b3839429eeaaca24689e998df1860b)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-efr32,openthread/ot-efr32,openthread/ot-efr32