Add npm ^9 as supported engines for dependabot

[eason9487]

Changes proposed in this Pull Request:

Add npm ^9 to facilitate dependabot upgrades npm package by opening PRs since dependabot is using an npm version that doesn't pair with Node.js 20.

💡 I will be skipping code review process as this change cannot be verified in local env and it's a developer-facing only change.

Checks:

• Does your code follow the WordPress coding standards?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changelog entry