Rename button "download torrent" to "download private torrent" when the tracker is in close mode

[josecelano]

Relates to: torrust/torrust-index#494

We renamed it when the tracker mode is: Private or PrivateWhitelisted

pub enum TrackerMode {
    // todo: use https://crates.io/crates/torrust-tracker-primitives
    /// Will track every new info hash and serve every peer.
    Public,
    /// Will only serve authenticated peers.
    Private,
    /// Will only track whitelisted info hashes.
    Whitelisted,
    /// Will only track whitelisted info hashes and serve authenticated peers.
    PrivateWhitelisted,
}

![image](https://github.com/torrust/torrust-index-gui/assets/58816/5f5b8778-3972-41f9-a924-54efb27cd140)