chore(discoveryengine): update dependencies

[angelcaamal]

Description

Fixes b/558839650
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 updates the Node engine requirement to >=20.0.0, upgrades @google-cloud/discoveryengine to ^2.0.0, and bumps chai to ^6.0.0. The review feedback correctly points out that upgrading chai to ^6.0.0 will break tests with an ERR_REQUIRE_ESM error because the package is configured as CommonJS, and suggests reverting chai to ^4.5.0.

[XrossFox]

LGTM