Replace Deprecated `serde_yaml` dependency

Lune depends on serde_yaml, however, this library seems to have been deprecated recently. Instead, we could use serde_yml, which seems to be an actively maintained forked of the above.

I think it would be wise to wait a while before making the switch to a different crate. serve_yaml hasn't had any issues affecting us, it was just deprecated/unmaintained because the author wanted to put time into their other projects instead. New crate is also going through rapid changes and seems to intentionally be at version 0.0.x