Update C++ SDK version number to 8.11.0. (#909)

Author: jonsimantov

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "8.10.0",
-  "stable": "8.10.0",
-  "head": "8.10.0"
+  "released": "8.11.0",
+  "stable": "8.11.0",
+  "head": "8.11.0"
 }

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
+### 8.11.0
 -   Changes
     -   Firestore/Database (Desktop): Upgrade LevelDb dependency to 1.23
         ([#886](https://github.com/firebase/firebase-cpp-sdk/pull/886)).