Update render.yaml

[Woody1193]

This PR addresses an issue wherein the reverse proxy cannot be deployed to Render because the plan for the associated private service is invalid.

Description

I changed the value for plan from free to starter for the private service.

Motivation and Context

The current version of the project cannot be deployed to Render because free plans are not supported for private services.

How Has This Been Tested?

I tested this by actually deploying the blueprint to Render, and noticed no error.

Checklist:

• [ ] My change requires a change to the documentation or CHANGELOG.
• [ ] I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.