Replacing fmt lib with std::format

[Woazboat]

Creating this issue mostly for documenting the challenges involved, since it came up as a potential change after switching to C++20.

Obviously depends on at least C++20 #323

• Replacing instances of fmt::format is trivial and just requires a rename to std::format
• FMT_COMPILE macros for optimized fmt format strings can simply be removed. std::format is already compile time.
• Some code snippets currently choose between different more optimized implementations depending on the version of the fmt lib. These could be simplified and replaced by the equivalent functions which are guaranteed to exist in std <format>.
• Custom fmt::formatter helpers (for e.g. the api06::id_version type) need to be changed to std::formatter.
• fmt::join() is currently used in several places to format ranges with separators ("1,2,3,4"). There is unfortunately no direct analogue available in C++20. Range formatting was not included in C++20 and was only introduced in C++23. It is also not yet supported by GCC libstdc++.
• std::format requires at least GCC 13, Debian stable still ships GCC 12

[Woazboat]

https://github.com/Woazboat/openstreetmap-cgimap/tree/c%2B%2B20_replace_fmt has all changes except a replacement for fmt::join

[mmd-osm]

A simplified replacement for fmt::join() might be good enough to format a few ids: https://cpp.godbolt.org/z/eovh9re8v

[Woazboat]

PR #474 should be revisited when support for GCC 12 is no longer required (see initial comment).