Add a changelog entry.

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+Deprecated: The Tensor and SymmetricTensor classes had functions
+`begin_raw()` and `end_raw()` that suggested that the elements of
+these objects are stored in arrays and consequently could be accessed
+consecutively through iterators. While they are indeed stored
+contiguously in memory, they are not part of an array and, as a
+consequence, C++ does not actually allow this kind of access. The
+functions have therefore been deprecated and will be removed in due
+time.
+<br>
+As a consequence, the `make_array_view()` functions that allow turning
+a Tensor or SymmetricTensor into an ArrayView object have now also
+been deprecated.
+<br>
+(Wolfgang Bangerth, 2021/08/19)