libstdc++: Remove dg-options "-std=gnu++20" from remaining 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/18_support/coroutines/95917.cc: Remove dg-options
	-std=gnu++2a.
	* testsuite/18_support/coroutines/hash.cc: Likewise.
	* testsuite/18_support/coroutines/lwg3460.cc: Likewise.
	* testsuite/18_support/destroying_delete.cc: Likewise.
	* testsuite/18_support/source_location/1.cc: Likewise.
	* testsuite/18_support/source_location/consteval.cc: Likewise.
	* testsuite/18_support/source_location/version.cc: Likewise.
	* testsuite/19_diagnostics/error_category/operators/three_way.cc:
	Likewise.
	* testsuite/19_diagnostics/error_code/operators/three_way.cc:
	Likewise.
	* testsuite/19_diagnostics/error_condition/operators/three_way.cc:
	Likewise.
	* testsuite/19_diagnostics/stacktrace/output.cc: Likewise.
	* testsuite/26_numerics/adjacent_difference/lwg2055.cc:
	Likewise.
	* testsuite/26_numerics/bit/bit.count/countl_zero.cc: Likewise.
	* testsuite/26_numerics/bit/bit.count/countr_one.cc: Likewise.
	* testsuite/26_numerics/bit/bit.count/countr_zero.cc: Likewise.
	* testsuite/26_numerics/bit/bit.count/popcount.cc: Likewise.
	* testsuite/26_numerics/bit/bit.pow.two/bit_ceil.cc: Likewise.
	* testsuite/26_numerics/bit/bit.pow.two/has_single_bit.cc:
	Likewise.
	* testsuite/28_regex/iterators/regex_iterator/lwg3719.cc:
	Likewise.
	* testsuite/28_regex/iterators/regex_token_iterator/lwg3719.cc:
	Likewise.
	* testsuite/28_regex/sub_match/compare_c++20.cc: Likewise.
	* testsuite/experimental/filesystem/iterators/106201.cc:
	Likewise.
	* testsuite/experimental/scopeguard/exit.cc: Likewise.
	* testsuite/experimental/scopeguard/uniqueres.cc: Likewise.