Update README.md removing spotify

[Misterodd]

Spotify is not supported anymore, but I also didn't want to add Apple Music, as a separate plugin is required that might not be installed. See navidrome/navidrome#5197
There may be a variant for usage with the config in navidrome.toml. Requiring the user to add:

Plugins.Enabled=true
Plugins.AutoReload=true
Agents="audiomuseai,deezer,lastfm"
DevArtistInfoTimeToLive="1s"

DevArtistInfoTimeToLive can also be set in the toml: https://github.com/navidrome/navidrome/blob/master/conf/configuration.go#L140

[gemini-code-assist]

Code Review

This pull request updates the README.md file to remove spotify from the ND_AGENTS environment variable list in the example services configuration. I have no feedback to provide as there were no review comments.