make contentsDeltaUri optional

[modular-magician]

Summary

Deleted required: true from contentsDeltaUri because it is not required property.

Background

• contentsDeltaUri, which is supposed to be optional, is currently required.
  • the official documentation states that it is possible to create an index without specifying contentsDeltaUri.
  • ref: https://cloud.google.com/vertex-ai/docs/vector-search/create-manage-index?hl=en#create-empty-index-batch
• It seems that the field was changed from optional to required in the following issue and pull request
  • issue: Error creating Index: googleapi: Error 400: contentsDeltaUri is required but missing from Index metadata. hashicorp/terraform-provider-google#15962
  • pull request: fix(vertexai): make contents_delta_uri a required field in google_vertex_ai_index magic-modules#9066
  • above issue mentions that the error occurs during terraform apply (without contentsDeltaUri).
  • when I switched to the version at that time (google-beta = "4.76.0") and ran terraform apply (without contentsDeltaUri), the error did not occur.
    • additionally, i confirmed that it is possible to create an index by making a request to the Vertex AI API without specifying contentsDeltaUri.
  • it seems likely that there was a bug in the Vertex API at the time.

Release Note

vertexai: made `contents_delta_uri` a optional field in `google_vertex_ai_index`

Derived from GoogleCloudPlatform/magic-modules#12637