Update content to remove redundant Robust math types (#38930)

Vector3, Vector4, Matrix4, and Quaternion are now gone. This switches us over to System.Numerics where applicable.

Content.Shared/Humanoid/SkinColor.cs

@@ -1,3 +1,4 @@
+using System.Numerics;
 using System.Security.Cryptography;
 using Microsoft.VisualBasic.CompilerServices;