added formerlyserializedas attribute for keypoints in templates

com.unity.perception/CHANGELOG.md

 
 All appearances of the term `KeyPoint` have been renamed to `Keypoint`. Therefore, the following steps will be required after upgrading to this version:
 * If you have code that relies on any renamed types or names, alter your code to reflect the new names.
-* If you are using custom `KeypointTemplate` assets besides the provided `CocoKeypointTemplate`, open those assets in a text editor and change the `keyPoints:` line to `keypoints:`, then save the file.
 
 ### Known Issues
 

com.unity.perception/Runtime/GroundTruth/Labelers/KeyPointTemplate.cs

 using System;
+using UnityEngine.Serialization;
 
 namespace UnityEngine.Perception.GroundTruth
 {
         /// <summary>
         /// Array of <see cref="KeypointDefinition"/> for the template.
         /// </summary>
+        [FormerlySerializedAs("keyPoints")]
         public KeypointDefinition[] keypoints;
         /// <summary>
         /// Array of the <see cref="SkeletonDefinition"/> for the template.