[Vertex AI] Rename `ImageGenerationResponse` to `ImagenGenerationResponse` #6179

Job	Run time
specs_checking	0s
update_SpecsTesting_repo	13s
buildup_SpecsTesting_repo_FirebaseCore	0s
buildup_SpecsTesting_repo	0s
abtesting_quickstart	0s
auth_quickstart	0s
crashlytics_quickstart	0s
database_quickstart	0s
dynamiclinks_quickstart	0s
firestore_quickstart	0s
inappmessaging_quickstart	0s
messaging_quickstart	0s
performance_quickstart	0s
remoteconfig_quickstart	0s
storage_quickstart	0s
	13s

Provide feedback