Update release version to 9.2.0. (#1008)

Author: jonsimantov

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "9.1.0",
-  "stable": "9.1.0",
-  "head": "9.1.0"
+  "released": "9.2.0",
+  "stable": "9.2.0",
+  "head": "9.2.0"
 }

release_build_files/readme.md

@@ -634,7 +634,7 @@ workflow use only during the development of your app, not for publicly shipping
 code.
 
 ## Release Notes
-### Upcoming Release
+### 9.2.0
 -   Changes
     - GMA: Added the Google Mobile Ads SDK with updated support for AdMob. See
       the [Get Started