Add MJX and bump version number to 3.0.0.

Co-authored-by: Baruch Tabanpour <btaba@google.com>
PiperOrigin-RevId: 574327508
Change-Id: Ia9b62fbc929c6869dfcec87636b2e10d405a1060

unity/Editor/Bindings/MujocoBinaryRetriever.cs

@@ -37,7 +37,7 @@ public class MujocoBinaryRetriever {
           if (AssetDatabase.LoadMainAssetAtPath(mujocoPath + "/mujoco.dylib") == null) {
             File.Copy(
                 "/Applications/MuJoCo.app/Contents/Frameworks" +
-                "/mujoco.framework/Versions/Current/libmujoco.2.3.8.dylib",
+                "/mujoco.framework/Versions/Current/libmujoco.3.0.0.dylib",
                 mujocoPath + "/mujoco.dylib");
             AssetDatabase.Refresh();
           }
@@ -45,7 +45,7 @@ public class MujocoBinaryRetriever {
           if (AssetDatabase.LoadMainAssetAtPath(mujocoPath + "/libmujoco.so") == null) {
             File.Copy(
                 Environment.GetFolderPath(Environment.SpecialFolder.UserProfile) +
-                "/.mujoco/mujoco-2.3.8/lib/libmujoco.so.2.3.8",
+                "/.mujoco/mujoco-3.0.0/lib/libmujoco.so.3.0.0",
                 mujocoPath + "/libmujoco.so");
             AssetDatabase.Refresh();
           }