chore(eventarc): update dependencies

[angelcaamal]

Description

Fixes b/559306081

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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 updates dependencies (express to ^5.0.0 and c8 to ^12.0.0) and Node.js engine requirements (>=20.0.0) across several Eventarc sample packages. Feedback was provided to also update the engines.node field in eventarc/generic/package.json to >=20.0.0 to maintain consistency and compatibility with the upgraded c8 package.

[amcolin]

LGTM