Add change note

Author: owen-mc

go/ql/lib/change-notes/2022-12-6-may-have-side-effects-return-stmt.md

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+---
+category: minorAnalysis
+---
+The definition of `mayHaveSideEffects` for `ReturnStmt` was incorrect when more
+than one expression was being returned. Such return statements were
+effectively considered to never have side effects. This has now been fixed.
+In rare circumstances `globalValueNumber` may have incorrectly treated two
+values as the same when they were in fact distinct.