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-chat/src/main/java/fr/pandacube/lib/chat/ChatColorGradient.java

@@ -22,6 +22,11 @@ public class ChatColorGradient {
 
     private final List<GradientColor> colors = new ArrayList<>();
 
+    /**
+     * Create the custom gradient.
+     */
+    public ChatColorGradient() {}
+
     /**
      * Put a specific color at a specific location in the gradient.
      * @param gradientLocation the location in the gradient.

pandalib-chat/src/main/java/fr/pandacube/lib/chat/ChatColorUtil.java

@@ -293,4 +293,7 @@ public class ChatColorUtil {
     }
 
 
+
+    private ChatColorUtil() {}
+
 }

pandalib-chat/src/main/java/fr/pandacube/lib/chat/ChatConfig.java

@@ -157,5 +157,9 @@ public class ChatConfig {
                     .thenText("] ");
         }
 
+        private PandaTheme() {}
+
     }
+
+    private ChatConfig() {}
 }

pandalib-chat/src/main/java/fr/pandacube/lib/chat/ChatStatic.java

@@ -679,5 +679,10 @@ public abstract class ChatStatic {
         return builder.style(c.style()).append(c.children());
     }
 
+    /**
+     * Creates a new {@link ChatStatic} instance.
+     */
+    protected ChatStatic() {}
+
 
 }

pandalib-chat/src/main/java/fr/pandacube/lib/chat/ChatUtil.java

@@ -657,5 +657,6 @@ public class ChatUtil {
         return str;
     }
 
+    private ChatUtil() {}
 
 }