`dbt_project.yaml` allows additional properties in `flags`, which can overlook typos in flag names

The [latest schema for dbt_project.yaml](https://github.com/dbt-labs/dbt-jsonschema/blob/1dfd5cb547015a82f8357b6757af7807c4e3b301/schemas/latest/dbt_project-latest.json#L64) doesn't have `additionalProperties: false` set for its top-level `flags` key. This means that if you make a typo on a flag name, your `dbt_project.yaml` will still pass JSON schema validation. Ask me how I know 😅

I'm happy to open a PR to fix this, but wanted to make sure that it's not intentional first, since `additionalProperties: false` is being used elsewhere in that schema.