libstdc++: Use constexpr instead of _GLIBCXX20_CONSTEXPR in <vector>

For the operator<=> overload we can use the 'constexpr' keyword
directly, because we know the language dialect is at least C++20.

libstdc++-v3/ChangeLog:

	* include/bits/stl_vector.h (operator<=>): Use constexpr
	instead of _GLIBCXX20_CONSTEXPR macro.