JS: align ui5-xss query severity with other XSS queries

Author: Napalys

javascript/frameworks/ui5/src/UI5Xss/UI5Xss.ql

@@ -4,7 +4,7 @@
  *              a cross-site scripting vulnerability.
  * @kind path-problem
  * @problem.severity error
- * @security-severity 6.1
+ * @security-severity 7.8
  * @precision high
  * @id js/ui5-xss
  * @tags security