docs(generativeai): add region tags for code samples

[msampathkumar]

Adding generativeaionvertexai_ region tags to replace old aiplatform_

In near future GenAI code samples, will only use generativeaionvertexai_ region tag prefix.

Description

Fixes #

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Checklist

• [ ] I have followed Contributing Guidelines from CONTRIBUTING.MD
• [ ] Tests pass: go test -v ./.. (see Testing)
• [ ] Code formatted: gofmt (see Formatting)
• [ ] Vetting pass: go vet (see Formatting)
• [ ] 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

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

- [aiplatform/snippets/embeddings.go:18](https://github.com/GoogleCloudPlatform/golang-samples/blob/93d8c9dbb195505f1244df9840013fc4e5f21f89/aiplatform/snippets/embeddings.go#L18), tag `generativeaionvertexai_text_embeddings` - [aiplatform/text-predict/text.go:18](https://github.com/GoogleCloudPlatform/golang-samples/blob/93d8c9dbb195505f1244df9840013fc4e5f21f89/aiplatform/text-predict/text.go#L18), tag `generativeaionvertexai_text_predictions`

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