fix: Removes side effect of setting skill mod when changing Owner (#1101)

## Breaking Change!
Setting `mod.Owner` will no longer update a skill mod.
**Please stick to the API and use `mobile.AddSkillMod(mod)` for all situations, including equipping.**


## Fixes
- [X] Fixes memory leak in factions.
- [X] Changes `List<SkillMod>` to `HashSet<SkillMod>`.
- [X] Eliminates skill mods adding/removing twice.

Projects/Server/Mobiles/Mods/MobileMod.cs

@@ -21,7 +21,7 @@ namespace Server;
 public partial class MobileMod
 {
     [DirtyTrackingEntity]
-    public virtual Mobile Owner { get; set; }
+    public Mobile Owner { get; set; }
 
     public MobileMod(Mobile owner) => Owner = owner;
 }