fix: Fixes poison field duration, cleans up skill calculations and poison calculations (#1582)

### Summary
- [X] Fixes poison field duration
- [X] Adds SA+ poison spell calculations
- [X] Cleans up skill calculations
- [X] Adjusts poison level thresholds to properly reflect OSI

Projects/UOContent/Spells/Ninjitsu/DeathStrike.cs

@@ -53,7 +53,7 @@ namespace Server.Spells.Ninjitsu
 
                 if (timer.Steps > 0)
                 {
-                    damageBonus = attacker.Skills.Ninjitsu.Fixed / 150;
+                    damageBonus = (int)(attacker.Skills.Ninjitsu.Value / 15);
                 }
 
                 timer.Stop();