prefer C++20 + fmt + string_view over boost::algorithm

• drop pre-C++20 code
• prefer C++20 and fmt over boost::algorithm
• add an implicit conversion from StringKey to std::string_view, so that StringKeyHash can be simplified
• reduce scope of operator+ where one of the arguments is a StringKey to its minimum, to avoid it being considered during overload resolution more than strictly necessary (note: as a result, lhcb/Rec!4527 (merged) should be merged first, as there was code relying on the implicit conversion to StringKey during concatenation)
• in Gaudi::Parsers::Utils::replaceEnvironments, look for the next replacement in the output string skipping over the already processed part, instead of replacing the first occurrence which would result in a problem if a previously substituted value would happen contain one of the next matches -- which probably never happens, except when crafting a malicious substitution which explicitly targets this problem...
• in PropertyValue, reuse the formatting defined in GaudiUtils for consistency instead of using its own in-situ formatting