Update issue templates (#3950)

Co-authored-by: Chris Elion <[email protected]>

Author: surfnerd

.github/ISSUE_TEMPLATE/bug_report.md

@@ -24,6 +24,7 @@ Please wrap in [triple backticks (```)](https://help.github.com/en/articles/crea
 If applicable, add screenshots to help explain your problem.
 
 **Environment (please complete the following information):**
+- Unity Version: [e.g. Unity 2020.1f1]
 - OS + version: [e.g. Windows 10]
 - _ML-Agents version_: (e.g. ML-Agents v0.8, or latest `develop` branch from source)
 - _TensorFlow version_: (you can run `pip3 show tensorflow` to get this)