Update evergreen config to no longer test against 1.8 driver

Author: alcaeus

.evergreen/config.yml

@@ -435,17 +435,13 @@ axes:
     display_name: Driver Version
     values:
       - id: "lowest-supported"
-        display_name: "1.8.1"
+        display_name: "1.9.0"
         variables:
-          DRIVER_VERSION: "1.8.1"
+          DRIVER_VERSION: "1.9.0"
       - id: "latest-stable"
         display_name: "1.9-stable"
         variables:
           DRIVER_VERSION: "stable"
-      - id: "1.8-dev"
-        display_name: "1.8-dev"
-        variables:
-          DRIVER_BRANCH: "v1.8"
       - id: "1.9-dev"
         display_name: "1.9-dev"
         variables: