Restore XssThroughDom.ql's severity

Author: rvermeulen

javascript/ql/src/Security/CWE-079/XssThroughDom.ql

@@ -4,7 +4,7 @@
  *              can lead to a cross-site scripting vulnerability.
  * @kind path-problem
  * @problem.severity warning
- * @security-severity 7.8
+ * @security-severity 6.1
  * @precision high
  * @id js/xss-through-dom
  * @tags security