squid: mark with known vulnerabilities

https://megamansec.github.io/Squid-Security-Audit/

It does not look like the vulnerabilities are going to be fixed anytime
soon.

pkgs/servers/squid/default.nix

···
     license = licenses.gpl2Plus;
     platforms = platforms.linux;
     maintainers = with maintainers; [ raskin ];
+    knownVulnerabilities = [ "Squid has multiple unresolved security vulnerabilities, for more information see https://megamansec.github.io/Squid-Security-Audit/" ];
   };
 }