Update libpqxx to 7.2.0

7.2.0
 - You can now implicitly convert a `const std::string &` to `zview`.
 - Replaced some overloads for C strings and C++ strings with `zview`.
 - Deprecating `binarystring`.  Use `std::basic_string<std::byte>` instead!
 - Array parser did not recognise escaping in unquoted values.
 - gcc10 test build fix: a result iterator is not the same thing as a `row`.
 - Doc fix: field size does _not_ include terminating zero.  (#356)
 - Fix error message in `demangle_type_name`: printed result, not raw name.
 - Fix compile warning in `demangle_type_name` on GNU systems.
 - Document that string conversions assume non-null values.
 - Start playing with C++20 _concepts._
 - Sketch out concepts-based `PQconnectdbParams` support.  (#343)
 - Add missing link to "datatypes" documentation.  (#346)
 - Supports `to_string`, `stream_to`, etc. for `binarystring`.  (#312)
 - Fixed infinite recursion when using `std::optional` in `stream_to`.  (#364)
 - Home-rolled hex-escaping.  Saves an allocation.
 - Catch floating-point negative overflow in `check_cast`, not underflow.
 - Bit more work on CMake build doc. (#318)
 - Typo in `datatypes.md`: `nullness`, not `nullness_traits`. (#353)
 - Fixed test names map in `tests/runner.cxx`. (#354)
 - Integral `from_string` now accept leading whitespace, as in composite types.
 - Experimental support basics for composite types.  (#355)
 - Use `stream_from` without knowing the number of fields.  (#357)
 - Global `size_buffer` function.
 - `quote()` now works for always-null types without conversions defined.
 - `std::nullopt` now converts to an SQL null.
 - Skip quoting and escaping array/composite fields of "safe" types.
 - New type trait: `is_unquoted_safe`.
 - Forbid invalid specialisations of `query_value`.
 - Fixed `mktemp` invocation that broke on FreeBSD.
 - Avoid unneeded encode/decode step on more binary data.
 - If `__cxa_demangle` fails, fall back on raw type name.  (#361)


(prlw1)