Python 3.12 base containers for Agent Platform Workbench custom containers

## Feature Feature **Python 3.12 base containers for Agent Platform Workbench custom containers** You can build [custom containers](https://cloud.google.com/gemini-enterprise-agent-platform/notebooks/workbench/instances/create-custom-container)for Agent Platform Workbench instances using Python 3.12 base containers, in addition to the default Python 3.10 base containers. The Python 3.12 standard and slim base containers are available at the following URIs: * `us-docker.pkg.dev/workbench-images/gcr.io/workbench-container-2606:latest` * `us-docker.pkg.dev/workbench-images/gcr.io/workbench-container-slim-2606:latest`