openapi: Change schema type for loaders and game_versions to string

[pavog]

The schema loaders and game_versions in the endpoint /project/{id|slug}/version are currently defined as:

        - in: query
          name: loaders
          required: false
          description: The types of loaders to filter for
          schema:
            type: array
            items:
              type: string
            example: '["fabric"]'
        - in: query
          name: game_versions
          required: false
          description: The game versions to filter for
          schema:
            type: array
            items:
              type: string
            example: '["1.18.1"]'

this is wrong.
It would mean that my request to list project versions would look like this:
https://api.modrinth.com/v2/project/Ojp3IEa8/version?loaders=paper&loaders=spigot&game_versions=1.20.1&game_versions=1.20.2
This does not work and returns this response:

{
  "error": "invalid_input",
  "description": "Error while validating input: Query deserialize error: duplicate field `loaders`"
}

This PR changes the schema type for loaders and game_versions to string.
It is actually a string, written like an array in json.

New

        - in: query
          name: loaders
          required: false
          description: The types of loaders to filter for
          schema:
            type: string
            example: '["fabric"]'
        - in: query
          name: game_versions
          required: false
          description: The game versions to filter for
          schema:
            type: string
            example: '["1.18.1"]'

And the request will look like this:
https://api.modrinth.com/v2/project/smithing-table-fix/version?loaders=["paper","spigot"]&game_versions=["1.20.1","1.20.2"]

Related to #138