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/networking/supybot.nix

@@ -103,6 +103,8 @@ in
         rm -f '${cfg.stateDir}/supybot.cfg.bak'
       '';
 
+      startLimitIntervalSec = 5 * 60;  # 5 min
+      startLimitBurst = 1;
       serviceConfig = {
         ExecStart = "${pyEnv}/bin/supybot ${cfg.stateDir}/supybot.cfg";
         PIDFile = "/run/supybot.pid";
@@ -110,8 +112,6 @@ in
         Group = "supybot";
         UMask = "0007";
         Restart = "on-abort";
-        StartLimitInterval = "5m";
-        StartLimitBurst = "1";
 
         NoNewPrivileges = true;
         PrivateDevices = true;