Only check UsePrivilegeSeparation if openssh < 7.5

OpenSSH hardcoded the `UsePrivilegeSeparation` option to `sandbox` in
7.5:

  Potentially-incompatible changes
  ================================

  This release includes a number of changes that may affect existing
  configurations:

   * This release deprecates the sshd_config UsePrivilegeSeparation
     option, thereby making privilege separation mandatory. Privilege
     separation has been on by default for almost 15 years and
     sandboxing has been on by default for almost the last five.

https://www.openssh.org/releasenotes.html

Let's only check it if the ssh version is applicable.

Author: rhmdnd

linux_os/guide/services/ssh/ssh_server/sshd_use_priv_separation/rule.yml

@@ -52,3 +52,4 @@ template:
         parameter: UsePrivilegeSeparation
         xccdf_variable: var_sshd_priv_separation
         datatype: string
+        obsolete_since_version: "7.5"