feat: Adds Mobile.Murderer virtual property, consolidates kill-threshold checks (#2355)

## Summary

- Adds `public virtual bool Murderer => Kills >= 5` property to `Mobile`, replacing ~30 scattered `Kills >= 5` / `Kills < 5` magic-number checks across the codebase
- `BaseCreature` overrides `Murderer` to also return `true` when `AlwaysMurderer` is set
- Updates `Corpse` serialization to v15, storing `_murderer` as a `bool` field (migrated from `int Kills >= 5` in earlier versions)

Projects/UOContent/Gumps/AdminGump.cs

@@ -1419,7 +1419,7 @@ namespace Server.Gumps
                     }
                 default:
                     {
-                        if (m.Kills >= 5)
+                        if (m.Murderer)
                         {
                             return 0x21;
                         }