docs: update gemini model

[twishabansal]

Description

Should include a concise description of the changes (bug or feature), it's
impact, along with a summary of the solution

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🛠️ Fixes #<issue_number_goes_here>

[gemini-code-assist]

Code Review

This pull request updates the Gemini model version in the JavaScript Genkit quickstart documentation from gemini-2.0-flash to gemini-2.5-flash. A review comment identifies that gemini-2.5-flash is an invalid model identifier and provides a suggestion to revert to a valid version to prevent runtime failures.