Fixes for gcc 13, clang 16 and C++20

• Fix warnings about implicit capture of this (deprecated in C++20)
• Fix possibly dangling reference to a temporary
• Fix deprecated enum arithmetic
• Remove unused variables
• Fix clang warning: unqualified call to std::move
• Replace std::random_shuffle with std::shuffle (random_shuffle was deprecated in C++14 and should have been removed in C++17)
• Fix class/struct discrepancy
• Hide spurious warning: possible dangling reference to temporary (see gaudi/Gaudi#278 (closed))

Note: when building *-gcc13-dbg platforms there are a number of spurious warnings in dictionaries, fixed by gaudi/Gaudi!1501 (merged)