Container Image URI in docs and README not consistent with official documentation

[maxhardt]

Problem
There seem to be two different container registries and as a result container image URIs in different parts of the vertex-ai documentation.

The official vertex-ai documentation for pre-built containers only contain {REGION}-docker.pkg.dev URIs. Example: us-docker.pkg.dev/vertex-ai/training/tf-cpu.2-2:latest

The examples described in this Python client library only contain gcr.io URIs (I assume the region is dynamically set based on the users location settings). Example: gcr.io/cloud-aiplatform/training/tf-cpu.2-2:latest

Solution
Align the documentation.