feat(generative ai): update the tuning example to use gemini-1.5 model

[liulehui]

1. move tuning namespace out of preview
2. shift from gemini-1.0 to gemini-1.5

Description

Fixes #

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Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[msampathkumar]

@liulehui - can you also please update the samples from #12605

Short Gist: In a couple of days, we are moving all the code samples into feature specific folder. More details @ go/runnable-snippets-plan

[liulehui]

@liulehui - can you also please update the samples from #12605

Short Gist: In a couple of days, we are moving all the code samples into feature specific folder. More details @ go/runnable-snippets-plan

Done! PTAL

[msampathkumar]

LGTM. Only updates in the imports & minor file updates.