libstdc++: Remove dg-options "-std=gnu++20" from 24_iterators tests

libstdc++: Remove dg-options "-std=gnu++20" from 24_iterators tests

The testsuite will automatically select C++20 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++23
and C++26 as well.

libstdc++-v3/ChangeLog:

	* testsuite/24_iterators/aliases.cc: Remove dg-options
	-std=gnu++2a.
	* testsuite/24_iterators/associated_types/incrementable.traits.cc:
	Likewise.
	* testsuite/24_iterators/associated_types/iterator.traits.cc:
	Likewise.
	* testsuite/24_iterators/associated_types/readable.traits.cc:
	Likewise.
	* testsuite/24_iterators/back_insert_iterator/constexpr.cc:
	Likewise.
	* testsuite/24_iterators/back_insert_iterator/pr93884.cc:
	Likewise.
	* testsuite/24_iterators/bidirectional/concept.cc: Likewise.
	* testsuite/24_iterators/common_iterator/1.cc: Likewise.
	* testsuite/24_iterators/common_iterator/100823.cc: Likewise.
	* testsuite/24_iterators/common_iterator/2.cc: Likewise.
	* testsuite/24_iterators/contiguous/concept.cc: Likewise.
	* testsuite/24_iterators/contiguous/tag.cc: Likewise.
	* testsuite/24_iterators/counted_iterator/1.cc: Likewise.
	* testsuite/24_iterators/counted_iterator/lwg3389.cc: Likewise.
	* testsuite/24_iterators/counted_iterator/lwg3643.cc: Likewise.
	* testsuite/24_iterators/customization_points/92894.cc:
	Likewise.
	* testsuite/24_iterators/customization_points/iter_move.cc:
	Likewise.
	* testsuite/24_iterators/customization_points/iter_swap.cc:
	Likewise.
	* testsuite/24_iterators/customization_points/lwg3420.cc:
	Likewise.
	* testsuite/24_iterators/forward/concept.cc: Likewise.
	* testsuite/24_iterators/front_insert_iterator/constexpr.cc:
	Likewise.
	* testsuite/24_iterators/front_insert_iterator/pr93884.cc:
	Likewise.
	* testsuite/24_iterators/headers/iterator/synopsis_c++20.cc:
	Likewise.
	* testsuite/24_iterators/indirect_callable/92894.cc: Likewise.
	* testsuite/24_iterators/indirect_callable/projected-adl.cc:
	Likewise.
	* testsuite/24_iterators/indirect_callable/projected.cc:
	Likewise.
	* testsuite/24_iterators/input/concept.cc: Likewise.
	* testsuite/24_iterators/insert_iterator/constexpr.cc: Likewise.
	* testsuite/24_iterators/istream_iterator/cons/sentinel.cc:
	Likewise.
	* testsuite/24_iterators/istream_iterator/sentinel.cc: Likewise.
	* testsuite/24_iterators/istreambuf_iterator/cons/sentinel.cc:
	Likewise.
	* testsuite/24_iterators/istreambuf_iterator/sentinel.cc:
	Likewise.
	* testsuite/24_iterators/move_iterator/cust.cc: Likewise.
	* testsuite/24_iterators/move_iterator/dr3435.cc: Likewise.
	* testsuite/24_iterators/move_iterator/input_iterator.cc:
	Likewise.
	* testsuite/24_iterators/move_iterator/lwg3390.cc: Likewise.
	* testsuite/24_iterators/move_iterator/lwg3391.cc: Likewise.
	* testsuite/24_iterators/move_iterator/move_only.cc: Likewise.
	* testsuite/24_iterators/move_iterator/p2520r0.cc: Likewise.
	* testsuite/24_iterators/move_iterator/rel_ops_c++20.cc:
	Likewise.
	* testsuite/24_iterators/move_iterator/sentinel.cc: Likewise.
	* testsuite/24_iterators/normal_iterator/cmp_c++20.cc: Likewise.
	* testsuite/24_iterators/output/concept.cc: Likewise.
	* testsuite/24_iterators/random_access/concept.cc: Likewise.
	* testsuite/24_iterators/range_access/range_access_cpp20.cc:
	Likewise.
	* testsuite/24_iterators/range_access/range_access_cpp20_neg.cc:
	Likewise.
	* testsuite/24_iterators/range_operations/100768.cc: Likewise.
	* testsuite/24_iterators/range_operations/advance.cc: Likewise.
	* testsuite/24_iterators/range_operations/advance_overflow.cc:
	Likewise.
	* testsuite/24_iterators/range_operations/distance.cc: Likewise.
	* testsuite/24_iterators/range_operations/lwg3392.cc: Likewise.
	* testsuite/24_iterators/range_operations/next.cc: Likewise.
	* testsuite/24_iterators/range_operations/prev.cc: Likewise.
	* testsuite/24_iterators/reverse_iterator/cust.cc: Likewise.
	* testsuite/24_iterators/reverse_iterator/dr3435.cc: Likewise.
	* testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc:
	Likewise.