fix: Vertex settings are ignored in example.env

What does this merge request do and why?

This MR fixes the issue in example.env that config.vertex_text_model.project and config.vertex_search.project are set to an empty value.

We comment out these environment variables so that it inherits from AIGW_GOOGLE_CLOUD_PLATFORM__PROJECT properly.

Related to fix: inject service account cred at application... (!807 - merged)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.