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/mail/dovecot.nix

@@ -427,12 +427,12 @@ in
       wantedBy = [ "multi-user.target" ];
       restartTriggers = [ cfg.configFile modulesDir ];
 
+      startLimitIntervalSec = 60;  # 1 min
       serviceConfig = {
         ExecStart = "${dovecotPkg}/sbin/dovecot -F";
         ExecReload = "${dovecotPkg}/sbin/doveadm reload";
         Restart = "on-failure";
         RestartSec = "1s";
-        StartLimitInterval = "1min";
         RuntimeDirectory = [ "dovecot2" ];
       };