chore(js/plugins/googleai): Completely remove deprecated googleai plugin plus

[ifielker]

…other chores:

• added a big deprecation warning for vertexai plugin from @genkit-ai/vertexai; (because some people are still using it)
• fixed a bug with imagen to allow falsy parameters like false and 0 (but still not undefined or null).

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]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses several maintenance tasks, including deprecating the @genkit-ai/vertexai plugin, fixing a bug in the imagen function to handle falsy parameters correctly, and removing the deprecated googleai plugin. These changes aim to improve code maintainability and guide users towards the recommended Google AI integration.

Highlights

• Deprecation Warning: Added a deprecation warning for the @genkit-ai/vertexai plugin, indicating its removal in a future release and recommending migration to @genkit-ai/google-genai.
• Bug Fix: Fixed a bug in the imagen function to allow falsy parameters like false and 0, while still excluding undefined or null.
• Code Removal: Completely removed the deprecated googleai plugin, including its source code, tests, and related files.

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[gemini-code-assist]

Code Review

This pull request includes changes to the google-genai and vertexai plugins, focusing on removing the deprecated googleai plugin and fixing a bug in the imagen parameter handling. The vertexai plugin also receives a deprecation warning. The changes in imagen.ts and converters.ts ensure that falsy parameters like false and 0 are correctly handled, while undefined and null values are removed. Tests are updated to reflect these changes.