Update Cmake version to 3.9

[jmklix]

Problem:

Fix CMake deprecation warnings: awslabs/aws-crt-cpp#606
CRT (Common Runtime) updated it's cmake version to 3.9 to fix this deprecation warnings. This also needs to be done for s2.tls

Solution:

update cmake version to 3.9

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

Test building s2n-tls with the latest version of cmake and not get any warnings

Out of scope:

any future cmake deprecation warnings will not be covered