Move away from deprecated percent() method.

Also fixed another small nit.

proxy/src/main/java/com/velocitypowered/proxy/util/bossbar/AdventureBossBarManager.java

@@ -215,8 +215,8 @@ public class AdventureBossBarManager implements BossBar.Listener {
       packet.setAction(com.velocitypowered.proxy.protocol.packet.BossBar.ADD);
       packet.setName(ProtocolUtils.getJsonChatSerializer(version).serialize(bar.name()));
       packet.setColor(COLORS_TO_PROTOCOL.get(bar.color()));
-      packet.setOverlay(bar.overlay().ordinal());
-      packet.setPercent(bar.percent());
+      packet.setOverlay(OVERLAY_TO_PROTOCOL.get(bar.overlay()));
+      packet.setPercent(bar.progress());
       packet.setFlags(serializeFlags(bar.flags()));
       return packet;
     }