Mention MEMORY64 in ChangeLog. NFC

From emscripten-core#22864

Author: sbc100

ChangeLog.md

@@ -20,6 +20,9 @@ See docs/process.md for more on how version tagging works.
 
 3.1.72 (in development)
 -----------------------
+- The `MEMORY64` setting no longer experimental. At time of writing all browsers
+  still require a flag to run the resulting binaries but that should change in
+  the coming months since the proposal is now at stage 4. (#22864)
 
 3.1.71 - 11/04/24
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