Update version to 'pre.1'

DevProject/Packages/packages-lock.json

       "depth": 0,
       "source": "local",
       "dependencies": {
-        "com.unity.ml-agents": "2.0.0-exp.1",
+        "com.unity.ml-agents": "2.0.0-pre.1",
         "com.unity.modules.physics": "1.0.0"
       }
     },

Project/Packages/packages-lock.json

       "depth": 0,
       "source": "local",
       "dependencies": {
-        "com.unity.ml-agents": "2.0.0-exp.1",
+        "com.unity.ml-agents": "2.0.0-pre.1",
         "com.unity.modules.physics": "1.0.0"
       }
     },

com.unity.ml-agents.extensions/package.json

   "unity": "2019.4",
   "description": "A source-only package for new features based on ML-Agents",
   "dependencies": {
-    "com.unity.ml-agents": "2.0.0-exp.1",
+    "com.unity.ml-agents": "2.0.0-pre.1",
     "com.unity.modules.physics": "1.0.0"
   }
 }

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [2.0.0-exp.1] - 2021-04-22
+## [2.0.0-pre.1] - 2021-04-22
 ### Major Changes
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
 - The minimum supported Unity version was updated to 2019.4. (#5166)

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 = "2.0.0-exp.1";
+        internal const string k_PackageVersion = "2.0.0-pre.1";
 
         const int k_EditorTrainingPort = 5004;
 

com.unity.ml-agents/package.json

 {
   "name": "com.unity.ml-agents",
   "displayName": "ML Agents",
-  "version": "2.0.0-exp.1",
+  "version": "2.0.0-pre.1",
   "unity": "2019.4",
   "description": "Use state-of-the-art machine learning to create intelligent character behaviors in any Unity environment (games, robotics, film, etc.).",
   "dependencies": {

utils/validate_versions.py

             f.write(new_contents)
 
     if csharp_version is not None:
-        package_version = f"{csharp_version}-exp.1"
+        package_version = f"{csharp_version}-pre.1"
         if csharp_extensions_version is not None:
             # since this has never been promoted we need to keep
             # it in preview forever or CI will fail