Default Nodejs Version has deprecation warning

Currently Node.js 18 is [the default](https://github.com/cloudfoundry/nodejs-buildpack/blob/05fbb1e80868dfa54dd7570f0296e7e29ed19594/manifest.yml#L5).
During staging, this results in the warning:
```
   **WARNING** node 18.x.x will no longer be available in new buildpacks released after 2025-04-30.
```

The [active Node.js version](https://nodejs.org/en/about/previous-releases) is 22.