ci(storagetransfer): update Node.js version to 20

[angelcaamal]

Description

Update the Node.js version from 16 to 20 in the storagetransfer GitHub Actions workflow.

Context

This PR updates the actions/setup-node step in the storagetransfer.yaml workflow to use Node.js 20. This change aligns this workflow with the rest of the repository's CI pipelines to maintain consistency. Additionally, upgrading allows us to take advantage of the latest features, security updates, and performance improvements available in Node.js 20.

Fixes Internal: b/549796667

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• 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 pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow 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

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[amcolin]

LGTM