Add aggregate to migration guide deprecations

modules/ROOT/pages/migration/index.adoc

@@ -564,3 +564,44 @@ A warning is raised if you use a deprecated value.
 === Deprecated aggregations on ID fields
 
 Aggregations on ID fields are now deprecated and will be removed in the future.
+
+
+=== Deprecated aggregation operations
+
+Explicit aggregation operations, both top-level and nested, have been deprecated in favor of the `aggregate` field inside connection operations.
+
+For example:
+
+[source, graphql, indent=0]
+----
+query {
+  moviesAggregate {
+    count
+    title {
+      shortest
+    }
+  }
+}
+----
+
+The previous query is deprecated in favor of the following:
+
+[source, graphql, indent=0]
+----
+query {
+  moviesConnection {
+    aggregate {
+      node {
+        title {
+          shortest
+        }
+      }
+      count {
+        nodes
+      }
+    }
+  }
+}
+----
+
+Note that `count` now returns the explicit fields `nodes` and `edges`.