feat!: support gemini-2.0-X, change default Gemini text generator model to gemini-2.0-flash-001, remove PaLM2TextGenerator and PaLM2TextEmbeddingGenerator

[shuoweil]

BEGIN_COMMIT_OVERRIDE
feat: support gemini-2.0-X models (#1558)

BREAKING CHANGE: change default LLM model to gemini-2.0-flash-001, drop PaLM2TextGenerator and PaLM2TextEmbeddingGenerator (#1558)
END_COMMIT_OVERRIDE

b/406270472 and b/406896634

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[GarrettWu]

Remove PaLM2TextGenerator and PaLM2TextEmbeddingGenerator as well

[snippet-bot]

Here is the summary of possible violations 😱

There are 3 possible violations for removing region tag in use.

• samples/snippets/text_generation_test.py:24, tag bigquery_dataframes_generate_text_tutorial_create_remote_model(usage: page 1).
• samples/snippets/text_generation_test.py:35, tag bigquery_dataframes_generate_text_tutorial_perform_keyword_extraction(usage: page 1).
• samples/snippets/text_generation_test.py:52, tag bigquery_dataframes_generate_text_tutorial_perform_sentiment_analysis(usage: page 1).

The end of the violation section. All the stuff below is FYI purposes only.

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You are about to delete the following sample browser page.

• samples/snippets/gen_ai_model_test.py:25, tag bigquery_dataframes_gen_ai_model(usage: page 1).

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Here is the summary of changes.

You are about to delete 4 region tags.

• samples/snippets/gen_ai_model_test.py:25, tag bigquery_dataframes_gen_ai_model
• samples/snippets/text_generation_test.py:24, tag bigquery_dataframes_generate_text_tutorial_create_remote_model
• samples/snippets/text_generation_test.py:35, tag bigquery_dataframes_generate_text_tutorial_perform_keyword_extraction
• samples/snippets/text_generation_test.py:52, tag bigquery_dataframes_generate_text_tutorial_perform_sentiment_analysis

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