Update version numbers, guids and readme for M59 6.8.0
Update version numbers, guids and readme for M59 6.8.0

PiperOrigin-RevId: 281640765

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk