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/web-servers/caddy.nix

@@ -101,6 +101,8 @@ in {
       after = [ "network-online.target" ];
       wants = [ "network-online.target" ]; # systemd-networkd-wait-online.service
       wantedBy = [ "multi-user.target" ];
+      startLimitIntervalSec = 14400;
+      startLimitBurst = 10;
       serviceConfig = {
         ExecStart = "${cfg.package}/bin/caddy run --config ${configJSON}";
         ExecReload = "${cfg.package}/bin/caddy reload --config ${configJSON}";
@@ -108,8 +110,6 @@ in {
         User = "caddy";
         Group = "caddy";
         Restart = "on-abnormal";
-        StartLimitIntervalSec = 14400;
-        StartLimitBurst = 10;
         AmbientCapabilities = "cap_net_bind_service";
         CapabilityBoundingSet = "cap_net_bind_service";
         NoNewPrivileges = true;