Add a changelog entry.

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+Changed: Several parts of the library involve interfacing with
+external libraries by way of user-defined callback functions. Specific
+examples are the interfaces to the SUNDIALS solvers (e.g., the
+SUNDIALS::KINSOL class). These interfaces typically required using the
+convention for error reporting defined by the underlying library -- in
+the case of SUNDIALS, for example, callbacks needed to return zero in
+case of success, a negative value for an irrecoverable error, and a
+positive value for a recoverable error.
+
+This approach does not scale across the many interfaces we have. As a
+consequence, we standardized how callbacks should behave, as
+documented in @ref GlossUserProvidedCallBack "this glossary entry". 
+The interfaces in SUNDIALS::KINSOL have been changed correspondingly.
+<br>
+(Wolfgang Bangerth, 2023/05/08)