refactor : Enum 변경 (Difficulty + 해당 난이도 점수정보 포함)

Author: Kimminu7

src/main/java/org/ezcode/codetest/domain/problem/model/enums/Difficulty.java

@@ -9,11 +9,13 @@
 
 @Getter
 public enum Difficulty {
-	BRONZE("브론즈", 20),
-	SILVER("실버", 40),
-	GOLD("골드", 60),
-	PLATINUM("플래티넘", 80),
-	DIAMOND("다이아", 100);
+	LV1("1", 10),
+	LV2("2", 20),
+	LV3("3", 40),
+	LV4("4", 80),
+	LV5("5", 160),
+	LV6("6", 320),
+	LV7("7",640);
 
 	private final String difficulty;