Update C++ SDK version to 8.8.0.

Author: jonsimantov

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "8.7.0",
-  "stable": "8.7.0",
-  "head": "8.7.0"
+  "released": "8.8.0",
+  "stable": "8.8.0",
+  "head": "8.8.0"
 }

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
-### Next Release
+### 8.8.0
 -   Changes
     -   General: Fixed a data race that could manifest as null pointer
         dereference in `FutureBase::Release()`.