Build Catch3 as C++17

C++17 is still the widest common target, and a library built on it stays
usable from C++17 and every later standard, while C++20 would have left
out many users.

Concepts become enable_if on named traits in Detail/TypeTraits.hpp, which
also replaces std::remove_cvref_t and std::identity. std::span becomes
std::vector references, std::bit_cast becomes static_cast and memcpy,
template lambdas become helper templates, and std::erase_if, contains,
starts_with and ends_with have their C++17 forms. Random::Between no
longer uses a lambda over a constexpr local, which MSVC rejects outside
/permissive-, so the headers work under its default mode too.

Tests give up designated initializers, which MSVC refuses in C++17. CI
builds the matrix in C++17 and C++20, since the headers promise both.

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