refactor(iam): update service account samples to use GAPIC library

[ssvir]

Description

Fixes

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Checklist

• [ ] I have followed Sample Format Guide
• [ ] pom.xml parent set to latest shared-configuration
• [ ] Appropriate changes to README are included in PR
• [ ] 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)
• [ ] Tests pass: mvn clean verify required
• [ ] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] 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

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[hegemonic]

I have a big-picture concern about this PR: Is someone also rewriting the other IAM code samples in this repo to use the GAPIC libraries?

If so, that's great! This migration is long overdue.

If not, I don't think we should merge this PR. It would be extremely confusing if some pages in the IAM docs told you to use the Google API Client Library for Java, and other pages told you to use the Cloud Client Library for Java. We need to update all of the samples for each language, not just some of them.

[m-strzelczyk]

Hey @hegemonic ! Thanks for your vigilance in this regard :) I'm happy to inform that it's a coordinated effort to move all Java and Python IAM samples that are still using the old libraries to the new ones. Please reach out to me if you're interested in more details.

[hegemonic]

@m-strzelczyk Fantastic! Very glad to hear it 😀

Can you let me know when you're mostly done with the IAM samples, so that we can update the client libraries page as well?