Add a changelog entry.

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+Changed: The two algorithms in the ConsensusAlgorithms namespace
+(along with their base class) required a callback function that
+correctly sized the buffer into which the answer to a request was to
+be written. These algorithms have been rewritten in ways that make
+this no longer necessary, and they now correctly size these buffers
+themselves. As a consequence, the ConsensusAlgorithms::Process class
+no longer has the `prepare_for_answer()` function, and the
+ConsensusAlgorithms::AnonymousProcess class no longer has a
+corresponding member variable; in the latter case, the constructor of
+the class also no longer takes an argument to this effect.
+<br>
+(Wolfgang Bangerth, Peter Munch, 2022/01/18)