Update release notes of Firebase CPP client to version 6.7.0

PiperOrigin-RevId: 278659164

Author: Googler

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "6.6.1",
-  "stable": "6.6.1",
-  "head": "6.6.1"
+  "released": "6.7.0",
+  "stable": "6.7.0",
+  "head": "6.7.0"
 }