Prevent web_hooks.recent_failures overflowing

What does this MR do and why?

Prior to this change, the recent_failures column would overflow if incremented repeatedly within WebHook#backoff! - this change clamps the value on all updates to this column.

See: #378081 (closed)

How to set up and validate locally

1. Enable the web-hooks-disable-failed flag:

   Feature.enable(:web_hooks_disable_failed)

2. Edit a web-hook model to have the maximum number of recent failures, as well as a defined disabled_until (the timestamp can be in the past):

   hook = WebHook.find(X)
   hook.update!(recent_failures: 32767, disabled_until: 1.hour.ago)

3. In the console try to backoff the hook:

   hook.backoff!

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.