Now that 4.0.0-rc0 is out, we need to add it to the evergreen testing, separate from testing "latest". This means copying the "qa-test" task to become "qa-test-3.6" and having "qa-test" use server 4.0.

Other tasks like "db" and "integration" that have mongo_version: "3.6" need to be updated to 4.0.