Use std::source_location in exceptions.

[jtv]

In libpqxx 8.0 we will assume compiler support for C++20. As of 7.10 we also have a streamlined system of SQL execution functions.

Combining those things it becomes more feasible to pass std::source_location in various places, and include it when throwing exceptions. This is what replaces the old "description" parameters. It won't be perfect from the start: the idea is to get the source location where libpqxx gets called from the outside, i.e. errors will indicate a location inside the caller's code.

[jtv]

(As a first step I'm retiring the checks and conditionals for source_location support: We can now just assume that this feature works.)