Add built-in cone mesh type.

Also delete `prism` type since it is subsumed by cone.

PiperOrigin-RevId: 790992343
Change-Id: I7a9c6f2a4377489173acd64b23086c748d2cee8e

unity/Runtime/Bindings/MjBindings.cs

@@ -501,11 +501,10 @@ public enum mjtMeshBuiltin : int{
   mjMESH_BUILTIN_NONE = 0,
   mjMESH_BUILTIN_SPHERE = 1,
   mjMESH_BUILTIN_HEMISPHERE = 2,
-  mjMESH_BUILTIN_PRISM = 3,
-  mjMESH_BUILTIN_CONE = 4,
-  mjMESH_BUILTIN_TORUS = 5,
-  mjMESH_BUILTIN_WEDGE = 6,
-  mjMESH_BUILTIN_PLATE = 7,
+  mjMESH_BUILTIN_CONE = 3,
+  mjMESH_BUILTIN_TORUS = 4,
+  mjMESH_BUILTIN_WEDGE = 5,
+  mjMESH_BUILTIN_PLATE = 6,
 }
 public enum mjtBuiltin : int{
   mjBUILTIN_NONE = 0,