build: use "20" for the default CMAKE_CXX_STANDARD
build: use "20" for the default CMAKE_CXX_STANDARD

Signed-off-by: Kefu Chai <cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>
Message-Id: <20220521102144.812433-2-cdc92d111bae0ed1ee3b3c8a1f849fcffc6b44f7@gmail.com>

avikivity/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar