chore(genai): update sentencepiece dependency

[XrossFox]

Description

To fix dependabot security alerts:

• Bump sentencepiece from 0.2.0 to 0.2.1 across generative_ai samples.

Fixes b/521868650

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.

Compliance & Style

• I have followed the Sample Guidelines.
• The README is updated with all relevant information (if applicable).
• If this PR adds a new sample directory: I updated the CODEOWNERS file.

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Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request updates the sentencepiece dependency from version 0.2.0 to 0.2.1 across multiple requirements.txt files in the generative_ai subdirectories, including embeddings, evaluation, extensions, image_generation, model_garden, model_tuning, prompts, and reasoning_engine. There are no review comments, and I have no additional feedback to provide.

[amcolin]

LGTM