whatsnew for websocket change

docs/source/whatsnew/pr/incompat-single-websocket.rst

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+* The notebook now uses a single websocket at `/kernels/<kernel-id>/channels` instead of separate
+  `/kernels/<kernel-id>/{shell|iopub|stdin}` channels. Messages on each channel are identified by a
+  `channel` key in the message dict, for both send and recv.