nixos/modules: fix systemd start rate-limits

These were broken since 2016:
https://github.com/systemd/systemd/commit/f0367da7d1a61ad698a55d17b5c28ddce0dc265a
since StartLimitIntervalSec got moved into [Unit] from [Service].
StartLimitBurst has also been moved accordingly, so let's fix that one
too.

NixOS systems have been producing logs such as:
/nix/store/wf98r55aszi1bkmln1lvdbp7znsfr70i-unit-caddy.service/caddy.service:31:
Unknown key name 'StartLimitIntervalSec' in section 'Service', ignoring.

I have also removed some unnecessary duplication in units disabling
rate limiting since setting either interval or burst to zero disables it
(https://github.com/systemd/systemd/blob/ad16158c10dfc3258831a9ff2f1a988214f51653/src/basic/ratelimit.c#L16)

nixos/modules/services/x11/xserver.nix

@@ -678,14 +678,14 @@ in
 
         script = "${cfg.displayManager.job.execCmd}";
 
+        # Stop restarting if the display manager stops (crashes) 2 times
+        # in one minute. Starting X typically takes 3-4s.
+        startLimitIntervalSec = 30;
+        startLimitBurst = 3;
         serviceConfig = {
           Restart = "always";
           RestartSec = "200ms";
           SyslogIdentifier = "display-manager";
-          # Stop restarting if the display manager stops (crashes) 2 times
-          # in one minute. Starting X typically takes 3-4s.
-          StartLimitInterval = "30s";
-          StartLimitBurst = "3";
         };
       };