CONTRIBUTING.md: bump to Cassandra 3.11.0
CONTRIBUTING.md: bump to Cassandra 3.11.0

criteo/biggraphite,Thib17/biggraphite,iksaif/biggraphite,iksaif/biggraphite,iksaif/biggraphite,Thib17/biggraphite,Thib17/biggraphite,criteo/biggraphite,Thib17/biggraphite,iksaif/biggraphite,criteo/biggraphite,criteo/biggraphite