Updated changelog [skip ci]

Author: ankane

CHANGELOG.md

@@ -3,7 +3,7 @@
 - Added support for libpqxx 8
 - Changed `HalfVector` to use `std::float16_t` or `_Float16` when available
 - Replaced `std::vector` conversion with `as_vector` function for `Vector` and `HalfVector`
-- Removed default constructors
+- Removed default constructors (no longer needed for streaming)
 - Dropped support for libpqxx 7
 - Dropped support for C++17