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/security/vault.nix

@@ -131,6 +131,8 @@ in
 
       restartIfChanged = false; # do not restart on "nixos-rebuild switch". It would seal the storage and disrupt the clients.
 
+      startLimitIntervalSec = 60;
+      startLimitBurst = 3;
       serviceConfig = {
         User = "vault";
         Group = "vault";
@@ -145,8 +147,6 @@ in
         KillSignal = "SIGINT";
         TimeoutStopSec = "30s";
         Restart = "on-failure";
-        StartLimitInterval = "60s";
-        StartLimitBurst = 3;
       };
 
       unitConfig.RequiresMountsFor = optional (cfg.storagePath != null) cfg.storagePath;