chore: add internal version note

Author: Timongcraft

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/VelocityServerConnection.java

@@ -67,7 +67,7 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
   private final VelocityServer server;
   private @Nullable MinecraftConnection connection;
   private boolean hasCompletedJoin = false;
-  private boolean clientLoaded = false;
+  private boolean clientLoaded = false; // 1.21.4+
   private boolean gracefulDisconnect = false;
   private BackendConnectionPhase connectionPhase = BackendConnectionPhases.UNKNOWN;
   private final Map<Long, Long> pendingPings = new HashMap<>();