update unity version

Author: Cynthia Jiang

cmake/firebase_unity_version.cmake

@@ -14,7 +14,7 @@
 
 # This file defines the version numbers used by the Firebase Unity SDK.
 
-set(FIREBASE_UNITY_SDK_VERSION "8.8.2"
+set(FIREBASE_UNITY_SDK_VERSION "8.8.1"
     CACHE STRING "The version of the Unity SDK, used in the names of files.")
 
 set(FIREBASE_IOS_POD_VERSION "8.10.0"