Update version number for 8.9.0 release. (#820)

Author: jonsimantov

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "8.8.0",
-  "stable": "8.8.0",
-  "head": "8.8.0"
+  "released": "8.9.0",
+  "stable": "8.9.0",
+  "head": "8.9.0"
 }

release_build_files/readme.md

@@ -567,12 +567,13 @@ workflow use only during the development of your app, not for publicly shipping
 code.
 
 ## Release Notes
-### Next Release
+### 8.9.0
 -   Changes
     -   General (iOS): Fixed an intermittent crash on iOS 15 caused by
         constructing C++ objects during Objective-C's `+load` method.
 	([#706](https://github.com/firebase/firebase-cpp-sdk/pull/706))
 	([#783](https://github.com/firebase/firebase-cpp-sdk/pull/783))
+    -   General: Internal changes to Mutex class.
 
 ### 8.8.0
 -   Changes