fix: register also validate for 5 length of password

Author: DuGuYifei

service-auth/src/main/java/de/tum/devops/auth/dto/RegisterRequest.java

@@ -26,7 +26,7 @@ public class RegisterRequest {
     private String email;
 
     @NotBlank(message = "Password is required")
-    @Size(min = 6, message = "Password must be at least 6 characters")
+    @Size(min = 5, message = "Password must be at least 5 characters")
     private String password;
 
     // Constructors