#601 bug: username is case sensitive, but email in general is not

Opted for completing remove support for uppercase letters in username
this way we never come across issued with casing in WebGoat

webgoat-container/src/main/java/org/owasp/webgoat/users/UserForm.java

@@ -17,7 +17,7 @@ public class UserForm {
 
     @NotNull
     @Size(min=6, max=20)
-    @Pattern(regexp = "[a-zA-Z0-9-]*", message = "can only contain letters, digits, and -")
+    @Pattern(regexp = "[a-z0-9-]*", message = "can only contain lowercase letters, digits, and -")
     private String username;
     @NotNull
     @Size(min=6, max=10)