libstdc++: Remove dg-options "-std=gnu++20" from 26_numerics tests

libstdc++: Remove dg-options "-std=gnu++20" from 26_numerics 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/26_numerics/accumulate/constexpr.cc: Remove
	dg-options -std=gnu++2a.
	* testsuite/26_numerics/accumulate/lwg2055.cc: Likewise.
	* testsuite/26_numerics/adjacent_difference/constexpr.cc:
	Likewise.
	* testsuite/26_numerics/bit/bit.cast/105027.cc: Likewise.
	* testsuite/26_numerics/bit/bit.cast/bit_cast.cc: Likewise.
	* testsuite/26_numerics/bit/bit.cast/version.cc: Likewise.
	* testsuite/26_numerics/bit/bit.count/countl_one.cc: Likewise.
	* testsuite/26_numerics/bit/bit.pow.two/bit_ceil_neg.cc:
	Likewise.
	* testsuite/26_numerics/bit/bit.pow.two/bit_floor.cc: Likewise.
	* testsuite/26_numerics/bit/bit.pow.two/bit_width.cc: Likewise.
	* testsuite/26_numerics/bit/bit.pow.two/lwg3656.cc: Likewise.
	* testsuite/26_numerics/bit/bit.rotate/rotl.cc: Likewise.
	* testsuite/26_numerics/bit/bit.rotate/rotr.cc: Likewise.
	* testsuite/26_numerics/bit/header-2.cc: Likewise.
	* testsuite/26_numerics/bit/header.cc: Likewise.
	* testsuite/26_numerics/complex/1.cc: Likewise.
	* testsuite/26_numerics/complex/2.cc: Likewise.
	* testsuite/26_numerics/complex/comparison_operators/more_constexpr.cc:
	Likewise.
	* testsuite/26_numerics/complex/operators/more_constexpr.cc:
	Likewise.
	* testsuite/26_numerics/complex/requirements/more_constexpr.cc:
	Likewise.
	* testsuite/26_numerics/complex/value_operations/more_constexpr.cc:
	Likewise.
	* testsuite/26_numerics/endian/1.cc: Likewise.
	* testsuite/26_numerics/endian/2.cc: Likewise.
	* testsuite/26_numerics/endian/3.cc: Likewise.
	* testsuite/26_numerics/exclusive_scan/constexpr.cc: Likewise.
	* testsuite/26_numerics/inclusive_scan/constexpr.cc: Likewise.
	* testsuite/26_numerics/inner_product/constexpr.cc: Likewise.
	* testsuite/26_numerics/inner_product/lwg2055.cc: Likewise.
	* testsuite/26_numerics/iota/constexpr.cc: Likewise.
	* testsuite/26_numerics/lerp/1.cc: Likewise.
	* testsuite/26_numerics/lerp/constexpr.cc: Likewise.
	* testsuite/26_numerics/lerp/version.cc: Likewise.
	* testsuite/26_numerics/midpoint/floating.cc: Likewise.
	* testsuite/26_numerics/midpoint/integral.cc: Likewise.
	* testsuite/26_numerics/midpoint/pointer.cc: Likewise.
	* testsuite/26_numerics/midpoint/pointer_neg.cc: Likewise.
	* testsuite/26_numerics/midpoint/version.cc: Likewise.
	* testsuite/26_numerics/numbers/1.cc: Likewise.
	* testsuite/26_numerics/numbers/2.cc: Likewise.
	* testsuite/26_numerics/numbers/3.cc: Likewise.
	* testsuite/26_numerics/numbers/float128.cc: Likewise.
	* testsuite/26_numerics/numbers/nonfloat_neg.cc: Likewise.
	* testsuite/26_numerics/partial_sum/constexpr.cc: Likewise.
	* testsuite/26_numerics/partial_sum/lwg2055.cc: Likewise.
	* testsuite/26_numerics/random/concept.cc: Likewise.
	* testsuite/26_numerics/reduce/constexpr.cc: Likewise.
	* testsuite/26_numerics/slice/compare.cc: Likewise.
	* testsuite/26_numerics/transform_exclusive_scan/constexpr.cc:
	Likewise.
	* testsuite/26_numerics/transform_inclusive_scan/constexpr.cc:
	Likewise.
	* testsuite/26_numerics/transform_reduce/constexpr.cc: Likewise.