Webhooks that were temporarily disabled were previously being disabled
for too long, too quickly.

When a webhook is temporarily disabled, it is disabled for a period of
time. If after it has auto-enabled the webhook receiver is still
returning a 5xx error code, the webhook is then temporarily disabled
again and for a longer period of time.

This period increases up until the maximum amount of time (the
`MAX_BACKOFF`), which is 1 day.

We have an optimisation that returns `MAX_BACKOFF` when we know there
have been a certain number of failed attempts.

This optimisation was configured according to previous logic, and was
now causing a webhook to reach the `MAX_BACKOFF` value too quickly.

Instead of the exponential backoff going:

1m, 2m, 4m, 8m, 16m, 32m, 1h 4m, 2h 8m, 4h 16m

It was instead going:

1m, 2m, 4m, 8m, 16m, 32m, 1h 4m, 2h 8m, 24h

Changelog: fixed