[TF2] Increased Half-Zatoichi duel damage to prevent survival in niche situations

[JoriKos]

When using the Battalion's Backup and while using the Resistance powerup, it is possible to just barely survive the Half-Zatoichi instant kill. I've increased the damage multiplier from 3 to 5. While multiplying the damage by 4 also does the trick, 5 is simply futureproofing it in case any of the values get tweaked or anything new comes along.

Fixes https://github.com/ValveSoftware/Source-1-Games/issues/6974

[allvei]

@JoriKos I suggest you use the same approach as the backstabs where we just ignore all resistances and continue dealing 3x damage regardless.