feat(csharp) .NET 8.0 code migration (#966)

* .net 8.0 migration

* update docs

* revert change

* revert change and update appendix docs

* remove static

* Update binary_search_insertion.cs

* Update binary_search_insertion.cs

* Update binary_search_edge.cs

* Update binary_search_insertion.cs

* Update binary_search_edge.cs

---------

Co-authored-by: Yudong Jin <krahets@163.com>

codes/csharp/chapter_searching/two_sum.cs

@@ -8,37 +8,37 @@ namespace hello_algo.chapter_searching;
 
 public class two_sum {
     /* 方法一：暴力枚举 */
-    public static int[] TwoSumBruteForce(int[] nums, int target) {
+    int[] TwoSumBruteForce(int[] nums, int target) {
         int size = nums.Length;
         // 两层循环，时间复杂度 O(n^2)
         for (int i = 0; i < size - 1; i++) {
             for (int j = i + 1; j < size; j++) {
                 if (nums[i] + nums[j] == target)
-                    return new int[] { i, j };
+                    return [i, j];
             }
         }
-        return Array.Empty<int>();
+        return [];
     }
 
     /* 方法二：辅助哈希表 */
-    public static int[] TwoSumHashTable(int[] nums, int target) {
+    int[] TwoSumHashTable(int[] nums, int target) {
         int size = nums.Length;
         // 辅助哈希表，空间复杂度 O(n)
-        Dictionary<int, int> dic = new();
+        Dictionary<int, int> dic = [];
         // 单层循环，时间复杂度 O(n)
         for (int i = 0; i < size; i++) {
             if (dic.ContainsKey(target - nums[i])) {
-                return new int[] { dic[target - nums[i]], i };
+                return [dic[target - nums[i]], i];
             }
             dic.Add(nums[i], i);
         }
-        return Array.Empty<int>();
+        return [];
     }
 
     [Test]
     public void Test() {
         // ======= Test Case =======
-        int[] nums = { 2, 7, 11, 15 };
+        int[] nums = [2, 7, 11, 15];
         int target = 13;
 
         // ====== Driver Code ======