Replaced the "sv_allowacsbanfunction" CVar with "sv_maxacsbanduration"

This is in response to the feedback left here: https://zandronum.com/tracker/view.php?id=4153#c23089

The reporter is suggesting a more dynamic approach for how long servers can allow players to be banned via ACS. Although we agreed from our previous discussions that no more than 60 minutes should be enough time to allow a mod to ban players, I have to agree that a CVar like sv_maxacsbanduration that lets the server fine-tune up to how long they want to allow mods to ban players sounds like much better solution.

This still offers the same level of security as the original sv_allowacsbanfunction CVar (i.e. the default value ensures that mods can't ban players unless enabled by the host, and ConsoleCommand can't change the value). However, the host may now choose to allow less than 60 minutes of ban time in case it's too long, or greater than that if they're comfortable with it. Ideally, a mod can also check GetCVar( "sv_maxacsbanduration" ) in ACS to know up to how long they can ban a player for.