IonQ uses incorrect path for backend lookup in v0.7.2

[Cynocracy]

This is mea culpa, but am cutting an issue to track it.

IonQ is using the path v0.1/jobs/backends to load dynamic backends, but this is on a deprecated API, and should instead be v0.2/backends.

[Cynocracy]

This issue was introduced in https://github.com/ProjectQ-Framework/ProjectQ/releases/tag/v0.7.2 AIUI