feat: truncate poll exponential backoff function to max 10s (#674)

Related to #346 and #380

The initial exponential back off algorithm introduced in #380 was not
truncated, which could lead to very long intervals. The recent release
of hcloud-go truncated the default exponential back off algorithm to
60s:
hetznercloud/hcloud-go@fd1f46c

If we take the scenario described in #346, I think we can still reduce
the max interval value for the exponential back off algorithm to <30s.

internal/app/app.go

@@ -143,7 +143,11 @@ func CreateHcloudClient(metricsRegistry *prometheus.Registry, logger log.Logger)
 	}
 
 	opts = append(opts, hcloud.WithPollOpts(hcloud.PollOpts{
-		BackoffFunc: hcloud.ExponentialBackoff(2, time.Duration(pollingInterval)*time.Second),
+		BackoffFunc: hcloud.ExponentialBackoffWithOpts(hcloud.ExponentialBackoffOpts{
+			Base:       time.Duration(pollingInterval) * time.Second,
+			Multiplier: 2,
+			Cap:        10 * time.Second,
+		}),
 	}))
 
 	return hcloud.NewClient(opts...), nil