Update version number to 13.0.0

Author: a-maurice

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "12.8.0",
-  "stable": "12.8.0",
-  "head": "12.8.0"
+  "released": "13.0.0",
+  "stable": "13.0.0",
+  "head": "13.0.0"
 }

release_build_files/readme.md

@@ -613,7 +613,7 @@ workflow use only during the development of your app, not for publicly shipping
 code.
 
 ## Release Notes
-### Upcoming Release
+### 13.0.0
 -   Changes
     - General (Android): Update to Firebase Android BoM version 34.0.0.
     - General (iOS): Update to Firebase Cocoapods version 12.0.0.