Deprecate logger and start moving to SLF4J

JDA already has SLF4J as a requirement, so we might as well use that instead of making our own.

src/main/java/wtf/beatrice/hidekobot/util/Logger.java

@@ -5,6 +5,7 @@ import java.time.format.DateTimeFormatter;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 
+@Deprecated
 public class Logger
 {