Replace internal Structs class with google-cloud-core version

[kolea2]

Just merged in googleapis/sdk-platform-java#2278, once it is released we should swap out any usages of https://github.com/googleapis/java-logging/blob/main/google-cloud-logging/src/main/java/com/google/cloud/logging/Structs.java with this upstream version instead.

[cindy-peng]

Looks like currently the change for google-cloud-core is still pending for release: googleapis/sdk-platform-java#2290

Will start the swap once the change has been released.