Schema rejects valid root-level prerelease-type option

`schemas/config.json` rejects a `prerelease-type` set at the root of the manifest config, even though it is a valid root option (it is defined in `ReleaserConfigOptions` and applied at the root via `allOf`, where it acts as the default for all packages).

#### Environment details

- OS: any
- Node.js version: n/a (schema validation in editor)
- npm version: n/a
- `release-please` version: schema on `main`

#### Steps to reproduce

1. Add `"prerelease-type": "alpha"` at the root of a `release-please-config.json` that references `schemas/config.json` via `$schema`.
2. Validate against the schema (e.g. in an editor with JSON schema support).
3. Validation fails with `Property prerelease-type is not allowed.`

The option works at runtime; only schema validation is affected.