rewrite the change-note to emphasise that the change is potentially breaking

Author: erik-krogh

javascript/ql/lib/change-notes/2022-04-04-dataflow-models.md

@@ -1,5 +1,6 @@
 ---
-category: minorAnalysis
+category: breaking
 ---
 * Many library models have been rewritten to use dataflow nodes instead of the AST.
-  These changes include some renamings where the old name is still available as a deprecated feature.
+  The types of some classes have been changed, and these changes may break existing code.
+  Other classes and predicates have been renamed, in these cases the old name is still available as a deprecated feature.