Turn on Flexible Collections v1 for self-host (#4253)

(cherry picked from commit 6646d11)

src/Core/Constants.cs

@@ -147,7 +147,8 @@ public static Dictionary<string, string> GetLocalOverrideFlagValues()
         return new Dictionary<string, string>()
         {
             { DuoRedirect, "true" },
-            { UnassignedItemsBanner, "true"}
+            { UnassignedItemsBanner, "true"},
+            { FlexibleCollectionsV1, "true" }
         };
     }
 }