RUBY-2909 add 5.0 as stress test version

Author: neilshweky

.evergreen/config/standard.yml.erb

@@ -184,7 +184,7 @@ buildvariants:
     matrix_spec:
       stress: on
       ruby: ["ruby-2.7"]
-      mongodb-version: ["4.4", '4.2', '4.0', '3.6']
+      mongodb-version: ['5.0', '4.4', '4.2', '4.0', '3.6']
       topology: replica-set
       os: ubuntu1604
     display_name: "${mongodb-version} ${topology} stress ${ruby}"