increment versions on the release branch (#3844)

com.unity.ml-agents/Runtime/Academy.cs

         /// Unity package version of com.unity.ml-agents.
         /// This must match the version string in package.json and is checked in a unit test.
         /// </summary>
-        internal const string k_PackageVersion = "0.15.1-preview";
+        internal const string k_PackageVersion = "1.0.0-preview";
 
         const int k_EditorTrainingPort = 5004;
 

com.unity.ml-agents/package.json

 {
   "name": "com.unity.ml-agents",
   "displayName": "ML Agents",
-  "version": "0.15.1-preview",
+  "version": "1.0.0-preview",
-}
+}

gym-unity/gym_unity/__init__.py

-__version__ = "0.16.0.dev0"
+__version__ = "0.16.0"

ml-agents-envs/mlagents_envs/__init__.py

-__version__ = "0.16.0.dev0"
+__version__ = "0.16.0"

ml-agents/mlagents/trainers/__init__.py

-__version__ = "0.16.0.dev0"
+__version__ = "0.16.0"