Update version to 12.1.0, and add readme for messaging fix.

Author: jonsimantov

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "12.0.0",
-  "stable": "12.0.0",
-  "head": "12.0.0"
+  "released": "12.1.0",
+  "stable": "12.1.0",
+  "head": "12.1.0"
 }

release_build_files/readme.md

@@ -642,6 +642,8 @@ code.
       [SDK documentation](https://developers.google.com/admob/cpp/sdk).
     - Messaging (Android): Fixed a potential race condition on receiving
       messages after cleanup.
+    - Messaging (iOS): Fixed an issue with notifications being lost if they
+      were received before Firebase Messaging was initialized.
 
 ### 12.0.0
 -   Changes