squid: remove knownVulnerabilities for 7.0+

In the past, it has been brought up that Squid had many security vulnerabilities
(see https://megamansec.github.io/Squid-Security-Audit/). As of version 7.0,
all of them have been solved, as tracked in their GitHub Security page:
https://github.com/squid-cache/squid/security

pkgs/by-name/sq/squid/package.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/"
-    ];
+    # In the past, it has been brought up that Squid had many security vulnerabilities
+    # (see https://megamansec.github.io/Squid-Security-Audit/). As of version 7.0,
+    # all of them have been solved, as tracked in their GitHub Security page:
+    # https://github.com/squid-cache/squid/security
+    knownVulnerabilities = [ ];
   };
 })