fix(compute): update nodemailer and remove decommissioned smtp-transport dependency

[angelcaamal]

Description

Fixes security vulnerabilities reported in dependencies by updating nodemailer to a secure version and removing the decommissioned nodemailer-smtp-transport package.

Changes Made:

• Updated nodemailer to the latest secure version to address security advisories.
• Removed the decommissioned nodemailer-smtp-transport dependency (npm audit fix).
• Refactored mailjet.js to use nodemailer's native SMTP transport configuration directly.
• Refactored unit tests in test/mailjet.test.js to align with the new native transport setup.

Fixes Internal: b/537368484

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]

Code Review

This pull request simplifies the Mailjet configuration by removing the deprecated nodemailer-smtp-transport dependency and passing options directly to nodemailer.createTransport. It also updates the corresponding unit tests. Feedback points out that the specified nodemailer version ^9.0.0 does not exist and will cause installation failures, and identifies a typo in the API key placeholder string.