update release notes and version json (#873)

- Updated the release notes with the pending release version number.
- Updated the version .json with the new version number.

Author: DellaBitta

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "8.9.0",
-  "stable": "8.9.0",
-  "head": "8.9.0"
+  "released": "8.10.0",
+  "stable": "8.10.0",
+  "head": "8.10.0"
 }

release_build_files/readme.md

@@ -567,7 +567,7 @@ workflow use only during the development of your app, not for publicly shipping
 code.
 
 ## Release Notes
-### Upcoming Release
+### 8.10.0
 -   Changes
     -   General (iOS): Fixed additional issues on iOS 15 caused by early
         initialization of Firebase iOS SDK.