refactor(Dict): 将 customKey 方法的访问权限修改为 protected

- 将 customKey 方法的访问权限从 private 修改为 protected
- 此修改可能旨在允许子类访问和重写该方法，增加代码的灵活性和可扩展性

hutool-core/src/main/java/cn/hutool/core/lang/Dict.java

@@ -653,7 +653,7 @@ public class Dict extends LinkedHashMap<String, Object> implements BasicTypeGett
 	 * @param key KEY
 	 * @return 小写KEY
 	 */
-	private String customKey(String key) {
+	protected String customKey(String key) {
 		if (this.caseInsensitive && null != key) {
 			key = key.toLowerCase();
 		}