DOC: add behavior change note

Author: tacaswell

doc/api/next_api_changes/behavior/16220-AL.rst

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+Canvas's callback registry now stored on Figure
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The canonical location of the `~.cbook.CallbackRegistry` used to
+handle Figure/Canvas events has been moved from the Canvas to the
+Figure.  This change should be transparent to almost all users,
+however if you are swapping switching the Figure out from on top of a
+Canvas or visa versa you may see a change in behavior.