fix: Make classes final and Node class static for checkstyle compliance

Author: Raghu0703

src/main/java/com/thealgorithms/datastructures/trees/AVLTree.java

@@ -7,12 +7,12 @@
  * 
  * @author Raghu0703
  */
-public class AVLTree {
+public final class AVLTree {
 
     /**
      * Node class representing each element in the AVL Tree
      */
-    class Node {
+   static class Node {
         int data;
         int height;
         Node left, right;

src/main/java/com/thealgorithms/datastructures/trees/BinarySearchTree.java

@@ -5,12 +5,12 @@
  * 
  * @author Raghu0703
  */
-public class BinarySearchTree {
+public final class BinarySearchTree {
 
     /**
      * Node class representing each element in the BST
      */
-    class Node {
+   static  class Node {
         int data;
         Node left, right;