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/monitoring/teamviewer.nix

@@ -31,14 +31,14 @@ in
       after = [ "NetworkManager-wait-online.service" "network.target" ];
       preStart = "mkdir -pv /var/lib/teamviewer /var/log/teamviewer";
 
+      startLimitIntervalSec = 60;
+      startLimitBurst = 10;
       serviceConfig = {
         Type = "forking";
         ExecStart = "${pkgs.teamviewer}/bin/teamviewerd -d";
         PIDFile = "/run/teamviewerd.pid";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
         Restart = "on-abort";
-        StartLimitInterval = "60";
-        StartLimitBurst = "10";
       };
     };
   };