Remove dependency on {fmt}

[php1ic]

We only make use of a small subset of the functionality of {fmt}, namely

• Printing to screen, or file as fmt::print() accepts any stream
• Converting various types to formatted strings

std::format does some of the above and std::print does the remainder but is part of C++23. Once compiler support exists for both, we will investigate migrating and removing the external dependency.

[php1ic]

Updates to ubuntu-latest in January 2025 means we now test with ubuntu 24.04.1, and use the default compilers, i.e. gcc13 and clang18.

The internet suggests that both of these support C++23 so let's see if this can be progressed.

[php1ic]

If the defaults don't work gcc-14 and clang-19 can be installed.