libstdc++: Remove dg-options "-std=gnu++20" from <compare> tests

libstdc++: Remove dg-options "-std=gnu++20" from <compare> 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/comparisons/algorithms/fallback.cc:
	Remove dg-options -std=gnu++2a.
	* testsuite/18_support/comparisons/algorithms/partial_order.cc:
	Likewise.
	* testsuite/18_support/comparisons/algorithms/strong_order.cc:
	Likewise.
	* testsuite/18_support/comparisons/algorithms/strong_order_floats.cc:
	Likewise.
	* testsuite/18_support/comparisons/algorithms/weak_order.cc:
	Likewise.
	* testsuite/18_support/comparisons/categories/94565.cc:
	Likewise.
	* testsuite/18_support/comparisons/categories/partialord.cc:
	Likewise.
	* testsuite/18_support/comparisons/categories/strongord.cc:
	Likewise.
	* testsuite/18_support/comparisons/categories/weakord.cc:
	Likewise.
	* testsuite/18_support/comparisons/categories/zero_neg.cc:
	Likewise.
	* testsuite/18_support/comparisons/common/1.cc: Likewise.
	* testsuite/18_support/comparisons/object/93479.cc: Likewise.
	* testsuite/18_support/comparisons/object/lwg3530.cc: Likewise.