Replace String::compose by std::format

[JanVogelsang]

This PR replaces our custom string compose functionality by the C++ standard format function introduced in C++20.

Quote @heplesser:

It seems we need to stick with std::format for some time, since GCC only supports std::format from v 13 onwards (https://en.cppreference.com/w/cpp/compiler_support/20) and our standard Linux runners have gcc 11.

[heplesser]

I am afraid that his will have to wait until we no longer support Ubuntu 22.04 with gcc-11 as default compiler.