[libc++][test] Clean-up `MinSequenceContainer` (#158432)
Follows-up #158344.
- Guard range-related functions with `TEST_STD_VER >= 23`.
- Mark range-related functions unconditionally `constexpr`.
- Add `TEST_CONSTEXPR_CXX20` to one `operator=`.
This will make `MinSequenceContainer` more consistent and useful for
legacy container adaptors or C++26 `constexpr` additions. Although we're
currently only using it with flat container adaptors.
