Update version numbers.

Author: jonsimantov

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "12.2.0",
-  "stable": "12.2.0",
-  "head": "12.2.0"
+  "released": "12.3.0",
+  "stable": "12.3.0",
+  "head": "12.3.0"
 }

release_build_files/readme.md

@@ -632,7 +632,9 @@ code.
 
 ## Release Notes
 ### Upcoming Release
--   Changes
+-   12.3.0
+    - General (Android): Update to Firebase Android BoM version 33.3.0.
+    - General (iOS): Update to Firebase Cocoapods version 11.2.0.
     - Messaging: Changed SetListener to send the last token received
       before the listener was set.
     - Remote Config: Fixed ConfigInfo fields to default to 0 when