Fix Javadoc warnings due to Java 21 update (+ some other warnings)

The default implicit constructor must also have a doc comment, so I have to make it explicit and either properly restrict the visibility of the constructor, or actually document it.

pandalib-ws/src/main/java/fr/pandacube/lib/ws/PayloadRegistry.java

@@ -100,4 +100,6 @@ public class PayloadRegistry {
         registerPayloadType("login-succeed", LoginSucceedPayload.class);
     }
 
+    private PayloadRegistry() {}
+
 }

pandalib-ws/src/main/java/fr/pandacube/lib/ws/payloads/LoginSucceedPayload.java

@@ -4,4 +4,9 @@ package fr.pandacube.lib.ws.payloads;
  * Payload used by the server in inform the client the login was successful.
  */
 public class LoginSucceedPayload extends Payload {
+
+    /**
+     * Creates a new {@link LoginSucceedPayload}.
+     */
+    public LoginSucceedPayload() {}
 }

pandalib-ws/src/main/java/fr/pandacube/lib/ws/payloads/Payload.java

@@ -5,4 +5,8 @@ package fr.pandacube.lib.ws.payloads;
  */
 public abstract class Payload {
 
+    /**
+     * Creates a new {@link Payload}.
+     */
+    public Payload() {}
 }