Help: Add missing release notes for genex $<REMOVE_DUPLICATES:list>

Help/release/dev/remove_duplicates.rst

@@ -0,0 +1,6 @@
+remove_duplicates
+-----------------
+
+* A new ``$<REMOVE_DUPLICATES:list>``
+  :manual:`generator expression <cmake-generator-expressions(7)>`
+  has been added.