Authz client not updated with the way of encoding the basic header

Closes keycloak#15086

authz/client/src/main/java/org/keycloak/authorization/client/Configuration.java

@@ -85,7 +85,7 @@ public void configureClientCredentials(Map<String, List<String>> requestParams,
                     throw new RuntimeException("Client secret not provided.");
                 }
 
-                requestHeaders.put("Authorization", BasicAuthHelper.createHeader(getResource(), secret));
+                requestHeaders.put("Authorization", BasicAuthHelper.RFC6749.createHeader(getResource(), secret));
             }
         };
     }