added cuGraph-DGL removal to gnn page

Author: acostadon

docs/cugraph-docs/source/graph_support/gnn_support.rst

@@ -2,6 +2,10 @@
 Graph Neural Network Support
 ============================
 
+**cuGraph-DGL has been removed as of release 25.06.** We recommend migrating to 
+cuGraph-PyG, which offers the same functionality along with additional features like support for heterogeneous sampling and a unified API. 
+The cuGraph team is not planning any further work in the DGL ecosystem going forward.
+
 Here is a talk that explains `Training GNNs at Internet Scale using cuGraph and WholeGraph <https://www.youtube.com/watch?v=rS-bcWm3jQo&t=2762s>`_