fix: make sure genkit core libs by default don't output debug messages

[pavelgj]

Running a simple app:

import { gemini15Flash, googleAI } from '@genkit-ai/googleai';
import { genkit } from 'genkit';

const ai = genkit({ plugins: [googleAI()] });

(async () => {
  const response = await ai.generate({
    model: gemini15Flash,
    prompt: 'tell me a joke',
  });

  console.log(response.text());
})();

$ tsx src/test.ts 
Why don't scientists trust atoms? 

Because they make up everything! 

Also fixed model deprecation warning.

Fixes #848

Checklist (if applicable):

• Tested (manually, unit tested, etc.)