Release Notes - C++ Driver - Version 3.6.4

Bug

• [CXX-2234] - Expect space after "base64" in extended JSON binary
• [CXX-2251] - Missing bsoncxx/types/value.hpp
• [CXX-2258] - Prelude headers applied incorrectly

Improvement

• [CXX-2211] - Remove usages of xtrace in shell scripts
• [CXX-2255] - Run Evergreen tasks on Ubuntu 18.04