nixos/modules: fix systemd start rate-limits

These were broken since 2016:
f0367da7d1
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
(ad16158c10/src/basic/ratelimit.c (L16))

nixos/modules/services/web-apps/moinmoin.nix

@@ -224,6 +224,8 @@ in
               chmod -R u+w ${dataDir}/${wikiIdent}/underlay
             '';
 
+            startLimitIntervalSec = 30;
+
             serviceConfig = {
               User = user;
               Group = group;
@@ -237,7 +239,6 @@ in
 
               Restart = "on-failure";
               RestartSec = "2s";
-              StartLimitIntervalSec = "30s";
 
               StateDirectory = "moin/${wikiIdent}";
               StateDirectoryMode = "0750";