CSHARP-5698: Fix failed atlas-search-index-helpers-test variant (#1750)

Author: sanych-sun

evergreen/evergreen.yml

@@ -1996,6 +1996,7 @@ task_groups:
             - "AWS_SESSION_TOKEN"
           env:
             CLUSTER_PREFIX: dbx-csharp-search-index
+            MONGODB_VERSION: "7.0"
           args:
             - ${DRIVERS_TOOLS}/.evergreen/atlas/setup-atlas-cluster.sh
       - command: expansions.update