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>

log/log.go

@@ -27,7 +27,6 @@ var redacted = &Hook{
 		// Keys from the config
 		"(ApiKey:\")[\\w]*",
 		"(Secret:\")[\\w]*",
-		"(Spotify.*ID:\")[\\w]*",
 		"(PasswordEncryptionKey:[\\s]*\")[^\"]*",
 		"(UserHeader:[\\s]*\")[^\"]*",
 		"(TrustedSources:[\\s]*\")[^\"]*",