Add a changelog entry.

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+Changed: We have always considered PETSc- and Trilinos-based vectors
+that have ghost elements as immutable, i.e., it is possible to read
+elements (including the ghost elements) but not to write into them. On
+the other hand, the `compress()` function available in all vector
+types is meant to communicate values written into non-locally-owned
+vector elements to their proper owners, for example during assembly of
+a right hand side vector. This `compress()` operation clearly only
+makes sense if a vector does not have ghost elements, because only
+then is it possible to write into the vector at all, but this
+restriction was not enforced -- the `compress()` function simply did
+not do anything at all in these cases. This has now changed: Because
+it does not make sense to call `compress()` on vectors that have ghost
+elements, it is now forbidden to call it and this case will be caught
+by an assertion.
+<br>
+(Wolfgang Bangerth, 2023/12/27)