chore(js/plugins/google-genai): deprecate some imagen and veo models

[ifielker]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request removes several deprecated imagen and veo models from the KNOWN_MODELS registry. However, the removal of imagen models contradicts the added documentation regarding backward compatibility, as it causes these models to inherit incorrect metadata capabilities from a generic model definition. It is recommended to keep these models in the registry but filter them from discovery to maintain consistent behavior.