remove built-in Spotify integration (#5197)

* refactor: remove built-in Spotify integration

Remove the Spotify adapter and all related configuration, replacing
the built-in integration with the plugin system. This deletes the
adapters/spotify package, removes Spotify config options (ID/Secret),
updates the default agents list from "deezer,lastfm,spotify" to
"deezer,lastfm", and cleans up all references across configuration,
metrics, logging, artwork caching, and documentation. Users with
Spotify config options will now see a warning that the options are
no longer available.

* feat: add ListenBrainz to list of default agents

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/artwork/reader_artist.go

@@ -79,7 +79,7 @@ func newArtistArtworkReader(ctx context.Context, artwork *artwork, artID model.A
 }
 
 func (a *artistReader) Key() string {
-	hash := md5.Sum([]byte(conf.Server.Agents + conf.Server.Spotify.ID))
+	hash := md5.Sum([]byte(conf.Server.Agents))
 	return fmt.Sprintf(
 		"%s.%t.%x",
 		a.cacheKey.Key(),