Bump the version to 3.4.0 ahead of the next release.

PiperOrigin-RevId: 818694877
Change-Id: I42e261956a0219e549bec6a54cbf330a74f02f9f

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.3.3.7.dylib",
+                "/mujoco.framework/Versions/Current/libmujoco.3.4.0.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-3.3.7/lib/libmujoco.so.3.3.7",
+                "/.mujoco/mujoco-3.4.0/lib/libmujoco.so.3.4.0",
                 mujocoPath + "/libmujoco.so");
             AssetDatabase.Refresh();
           }