add a changes file for 25268

changes/ticket25268

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+  o Removed features:
+    - The old "round-robin" circuit multiplexer (circuitmux)
+      implementation has been removed, along with a fairly large set of
+      code that existed to support it.  It has not been the default
+      circuitmux since we introduced the "EWMA" circuitmux in 0.2.4.x,
+      but it still required an unreasonable amount of memory and CPU.
+      Closes ticket 25268.