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/nix-store-gcs-proxy.nix

@@ -42,9 +42,9 @@ in
       description = "A HTTP nix store that proxies requests to Google Storage";
       wantedBy = ["multi-user.target"];
 
+      startLimitIntervalSec = 10;
       serviceConfig = {
         RestartSec = 5;
-        StartLimitInterval = 10;
         ExecStart = ''
           ${pkgs.nix-store-gcs-proxy}/bin/nix-store-gcs-proxy \
             --bucket-name ${cfg.bucketName} \