Add a changelog entry.

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+Changed: Mapping::transform_points_real_to_unit_cell() used to
+designate points for which it could not successfully find reference
+coordinates by setting the first vector component to
+`std::numeric_limits<double>::infinity()`. Unfortunately, on some
+platforms, the use of infinities leads to dramatically slower code
+execution. As a consequence, we now use the (finite) value
+`std::numeric_limits<double>::lowest()` (somewhere around `-1e308`) to
+denote invalid values.
+<br>
+(Wolfgang Bangerth, 2024/12/17)