Log Message:
Update libpqx to 7.7.4

You now have three basic ways to query data, all through methods on your \ 
transaction object:

 - "query" methods execute the query, load the full result set,
    and then let you iterate the rows. They convert each row to a
   tuple of values of your chosen types.

 - "stream" methods do the same thing, except they run the query
   in a different way that's slower to start up, but starts giving
   you rows right away without waiting for the full result to come
   in. The rows also come in faster.

 - "exec" methods execute the query, load the full result set,
   and return a pqxx::result object. It contains both the result
   data and metadata such as how many rows there are in the result.

Highlights:
 - Build docs in `doc/html/`, no longer in `doc/html/Reference/`.
 - New `result::for_stream()`: simple iteration and conversion of rows.
 - New `transaction_base::for_query()` is similar, but non-streaming. (#580)
 - Query data and iterate directly as client-side types: `query()`. (#580)
 - New ways to query a single row!  `query01()` and `query1()`. (#580)
 - We now have 3 kinds of execution: "exec", "query", and \ 
"stream" functions.
 - Use C++23 `std::unreachable()` where available.
 - New class, `connecting` for nonblocking connection to the database.  (#487)
 - New class, `range` for SQL range types.  (#490)
 - Support `result::at(row_num, col_num)`.
 - Support `result[row_num, col_num]` if the compiler allows it.
 - `result::iter()` return value now keeps its `result` alive.
 - Add C++20 concepts: `binary`, `char_string`, `char_strings`.
 - New helper: `pqxx::value_type<CONTAINER>`.
 - New helper: `pqxx::binary_cast`. (#450)
 - Helper for generating parameter placeholders `$1`, `$2`, etc.  (#443)
 - Now requires `std::variant` support!  No longer works with gcc7.
 - New `blob::read()` using `std::span`.  (#429)
 - New, simpler API for large objects: `blob` ("binary large object").

For full changelog, see https://github.com/jtv/libpqxx/blob/7.7.4/NEWS