libstdc++: Use feature test macros consistently in <bits/stl_iterator.h>

libstdc++: Use feature test macros consistently in <bits/stl_iterator.h>

Remove __cplusplus > 201703L checks that are redundant when used
alongside __glibcxx_concepts checks, because <version> already
guarantees that __glibcxx_concepts is only defined for C++20 and later.

Prefer to check __glibcxx_ranges for features such as move_sentinel that
were added by the One Ranges proposal (P0896R4), or for features which
depend on other components introduced by that proposal.

But prefer to check __glibcxx_concepts for constraints that only depend
on requires-clauses and concepts defined in <concepts>, even if those
constraints were added by the Ranges proposal (e.g. the constraints on
non-member operators for move_iterator).

Prefer #ifdef to #if when just testing for the presence of __glibcxx_foo
macros with caring about their value.

Also add/tweak some Doxygen comments.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h: Make use of feature test macros
	more consistent. Improve doxygen comments.