httpd: Don't require keys.target

This has the unintended side-effect of restarting httpd every time we
run switch-to-configuration, even if httpd hasn't changed (because
we're doing a "stop keys.target" now).  So use a "Wants" dependency
instead.

nixos/modules/services/web-servers/apache-httpd/default.nix

@@ -621,7 +621,7 @@ in
       { description = "Apache HTTPD";
 
         wantedBy = [ "multi-user.target" ];
-        requires = [ "keys.target" ];
+        wants = [ "keys.target" ];
         after = [ "network.target" "fs.target" "postgresql.service" "keys.target" ];
 
         path =