Update version numbers, guids and readme for M59 6.8.0

PiperOrigin-RevId: 281640765

Author: chkuang-g

cpp_sdk_version.json

@@ -1,5 +1,5 @@
 {
-  "released": "6.7.0",
-  "stable": "6.7.0",
-  "head": "6.7.0"
+  "released": "6.8.0",
+  "stable": "6.8.0",
+  "head": "6.8.0"
 }