consistently use LHCb::span instead of gsl::span

avoid mixing LHCb::span (which defers to gsl::span) and direct use of gsl::span, as LHCb::span makes sure several macros are defined to avoid bounds checking in non-debug builds, and ensures that span<byte> uses std::byte instead of gsl::byte, and we don't want to have two 'variants' of gsl::span being used simultaneously which are configured differently...

(and it will make a future migration to C++20 std::span easier as well)

Closes #351