[OpenAPI] Add missing info.description

## 🚀 Feature Proposal Add missing `info.description` to the OpenAPI generator ## Motivation We currently add descriptions via an overlay but would like to have that come from the specification instead. ## Example This is the current content that is added by the overlay: ```json "description": "Elasticsearch provides REST APIs that are used by the UI components and can be called directly to configure and access Elasticsearch features.\n## Documentation source and versions\nThis documentation is derived from the `main` branch of the [elasticsearch-specification](https://github.com/elastic/elasticsearch-specification) repository. It is provided under license [Attribution-NonCommercial-NoDerivatives 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/).\nThis documentation contains work-in-progress information for future Elastic Stack releases.\n", ```