update c# version to 2.0.0-exp.1 (#5160)

Co-authored-by: Chris Elion <chris.elion@unity3d.com>

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

   "unity": "2018.4",
   "description": "A source-only package for new features based on ML-Agents",
   "dependencies": {
-    "com.unity.ml-agents": "1.9.0-preview"
+    "com.unity.ml-agents": "2.0.0-exp.1"
   }
 }

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 = "1.9.0-preview";
+        internal const string k_PackageVersion = "2.0.0-exp.1";
 
         const int k_EditorTrainingPort = 5004;
 

com.unity.ml-agents/package.json

 {
   "name": "com.unity.ml-agents",
   "displayName": "ML Agents",
-  "version": "1.9.0-preview",
+  "version": "2.0.0-exp.1",
   "unity": "2018.4",
   "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 = csharp_version + "-preview"
+        package_version = f"{csharp_version}-exp.1"
-            extension_version = csharp_extensions_version + "-preview"
+            # since this has never been promoted we need to keep
+            # it in preview forever or CI will fail
+            extension_version = f"{csharp_extensions_version}-preview"
         print(
             f"Setting package version to {package_version} in {MLAGENTS_PACKAGE_JSON_PATH}"
             f" and {MLAGENTS_EXTENSIONS_PACKAGE_JSON_PATH}"