Fixes for LCG 104

LCG 104 will have a number of changes including fmtlib 10, new ROOT and C++20 (with gcc13). This require adaptations:

• fixes for fmtlib (see gaudi/Gaudi!1462 (merged))
• fixes for C++20
  • fix order between forward declarations and use
  • fix error: template-id not allowed for destructor
  • add missing default constructor
• workaround for using namespace std; in TMVAImpFactory.h
• change needed to work with LHCb!4143 (merged)

Requires LHCb!4143 (merged)

Note: this is part of a pool of related (mostly not interdependent) MRs: Detector!400 (merged), LHCb!4143 (merged), Run2Support!46 (merged), Boole!483 (merged), !3449 (merged), Alignment!388 (merged)

See also https://its.cern.ch/jira/projects/LHCBPS/issues/LHCBPS-1912