Reduce severity of java/relative-path-command

Significantly reduces the severity of `java/relative-path-command` from 9.8 to 5.4

https://www.first.org/cvss/calculator/4.0#CVSS:4.0/AV:L/AC:L/AT:P/PR:H/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Author: JLLeitschuh

java/ql/src/Security/CWE/CWE-078/ExecRelative.ql

@@ -4,7 +4,7 @@
  *              malicious changes in the PATH environment variable.
  * @kind problem
  * @problem.severity warning
- * @security-severity 9.8
+ * @security-severity 5.4
  * @precision medium
  * @id java/relative-path-command
  * @tags security