C++20 fixes

Fix compilation with gcc13 / c++20 , ROOT 6.28.04 (LCG dev4), fmtlib 10.0.0

• use fmt::runtime where needed (with a hack to still work with fmtlib < 8)
• avoid implicit capture of this in lambdas via [=]
• fix comparison between Gaudi::Property<std::string> and const char*
• hide std::source_location to cling

Closes #266 (closed)