Potential issue with the JSON Schema

#### Environment details

- OS: any (MacOS / Linux)
- Node.js version: any (v20)
- npm version: any
- `release-please` version: latest

#### Steps to reproduce

A schema that contains incorrect properties isn't reported, e.g.

```
{
"$schema": "https://raw.githubusercontent.com/googleapis/release-please/refs/heads/main/schemas/config.json",
"packages": {
".": {
"application-type": "simple",
"extra-files": ["README.md"],
"other-non-existent-property": "no-errors"
}
}
}
```

doesn't report issues.

This is because `ReleaserConfigOptions` doesn't have `"additionalProperties": false` in its specs.

Adding this field would cause conflicts with `allOf`, so that needs to be changed to `anyOf` as well to allow for both single-package configs and multi-package configs