Allow changing the torrent category

[josecelano]

Now, it's impossible to change the torrent category after uploading it.

https://github.com/torrust/torrust-index-backend/blob/develop/src/web/api/v1/contexts/torrent/forms.rs#L6-L10

pub struct UpdateTorrentInfoForm {
    pub title: Option<String>,
    pub description: Option<String>,
    pub tags: Option<Vec<TagId>>,
}

This is needed to implement this issue in the frontend.