Handle Gemini Deprecation

[sragss]

gemini-2.0-flash-preview-image-generation and gemini-2.5-flash-image-preview are being retired.

https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/2-0-flash

Remove these from the docs, the router and the SDK.

[Trynax]

/attempt #634

[0xOlivanode]

Hey @sragss please could you assign this issue to me?

[285729101]

Addressed this in #720 — removed the deprecated preview image models and updated the defaults to their stable replacements.