Update the C++ and Unity versions to 5.7.0, including release notes

PiperOrigin-RevId: 243888508

Author: a-maurice

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "5.6.1",
-  "stable": "5.6.1",
-  "head": "5.6.1"
+  "released": "5.7.0",
+  "stable": "5.7.0",
+  "head": "5.7.0"
 }