Python: Add security-severity for py/insecure-cookie

Matching the Java query
https://github.com/github/codeql/blob/7d4767a4f57316429dda8813e6c744ecd4f16cb5/java/ql/src/Security/CWE/CWE-614/InsecureCookie.ql#L7

Author: RasmusWL

python/ql/src/experimental/Security/CWE-614/InsecureCookie.ql

@@ -4,6 +4,7 @@
  *              interception.
  * @kind problem
  * @problem.severity error
+ * @security-severity 5.0
  * @id py/insecure-cookie
  * @tags security
  *       external/cwe/cwe-614