Use standard C23 and C++ attributes if available

Use standard C23 and C++ attributes if available

Use the standard C23 and C++ attributes [[nodiscard]], [[noreturn]],
and [[maybe_unused]], if available.

This makes pg_nodiscard and pg_attribute_unused() available in
not-GCC-compatible compilers that support C23 as well as in C++.

For pg_noreturn, we can now drop the GCC-specific and MSVC-specific
fallbacks, because the C11 and the C++ implementation will now cover
all required cases.

Note, in a few places, we need to change the position of the attribute
because it's not valid in that place in C23.

Discussion: https://www.postgresql.org/message-id/flat/pxr5b3z7jmkpenssra5zroxi7qzzp6eswuggokw64axmdixpnk@zbwxuq7gbbcw