Switch from fmtlib to std::format.

[Holt59]

See ModOrganizer2/modorganizer-uibase#143

I removed all the manual call to toStdString() after tr, that's on purpose - Since std::string is convertible to std::format_string, that would trigger the format_string overload, but since the expression would not be compile time, fail.

Supersedes #2027

Others:

• Remove libffi from dependencies in Github action.