Java: Add change note.

Author: aschackmull

java/change-notes/2021-06-01-collection-flow.md

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+lgtm,codescanning
+* Data flow now tracks steps through collections and arrays more precisely.
+  That means that collection and array read steps are now matched up with
+  preceding store steps. This results in increased precision for all flow-based
+  queries, in particular most of the security queries.