Add a changelog entry.

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+Removed: Some, but not all, of the vector classes were derived from a
+base class VectorSpaceVector. This class had been intended to provide
+an abstract interface (via `virtual` functions) to vector-vector
+operations such as dot products or norms. But it turns out that that
+is not practical in many cases: Functions still need to either have
+access to individual elements of the vector, or they need to be able
+to do matrix-vector products. As a consequence, it is rarely useful to
+only have a reference to the base class VectorSpaceVector: One
+actually needs a reference to the derived class. Because of this lack
+of use, we have removed the VectorSpaceVector base class from the
+library.
+<br>
+(Wolfgang Bangerth, 2023/07/06)