fix(functions): WIP update and fix `functions_billing_stop` sample

[eapl-gemugami]

Description

Fixes b/403855924

This sample for a Cloud Run Function used in disable-billing-with-notifications#create-cloud-run-function is updated and validated to current Node 20 requirements.

Checklist

• [x] I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• [x] Tests pass: npm test (see Testing)
• [x] Lint pass: npm run lint (see Style)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones) <- I'm not sure yet on how to test this sample, but I'll work on top of the previous sample
• [x] 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 <- It creates a subdirectory, but I haven't run this step yet
• [ ] This sample adds a new sample directory, and I created GitHub Actions workflow for this sample <- It creates a subdirectory, but I haven't run this step yet
• [ ] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

• functions/v2/stopBilling/index.js:15, tag functions_billing_stop

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[iennae]

@davidcavazos if you could also take a look at this one; thanks!

[eapl-gemugami]

@davidcavazos for this sample I'm planning to add a Unit Test similar to that on Python stop_billing_test.py

The current one is just a copy from the test in the parent folder.

[eapl-gemugami]

@davidcavazos any comments on this approach of making an Unit Test based on the Python sample? Thanks!

[eapl-gemugami]

@glasnt thanks a lot for your review! I'll work on those comments I have given a thumbs up to, and I have a few questions in the replies.