chore(deps): update actions/checkout action to v4 (non-fork)

[glasnt]

Description

Replaces #3546, replicating changes on a branch rather than a fork.

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)
• 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

[rogerthatdev]

the functions-slack test has consistently been failing and currently blocking submitting this PR.

@glasnt Would you remove the change to .github/workflows/functions-slack.yaml from this PR and put it in another one?

[rogerthatdev]

All tests pass, LGTM!