libstdc++: deprecate is_trivial for C++26 (P3247R2)

This actually implements P3247R2 by deprecating the is_trivial type
trait.

libstdc++-v3/ChangeLog:

	* include/std/type_traits: Deprecate is_trivial and
	is_trivial_v.
	* include/experimental/type_traits: Suppress the deprecation
	warning.
	* testsuite/20_util/is_trivial/requirements/explicit_instantiation.cc:
	Amend the test to suppress the deprecation warning.
	* testsuite/20_util/is_trivial/requirements/typedefs.cc:
	Likewise.
	* testsuite/20_util/is_trivial/value.cc: Likewise.
	* testsuite/20_util/variable_templates_for_traits.cc: Likewise.
	* testsuite/experimental/type_traits/value.cc: Likewise.
	* testsuite/18_support/max_align_t/requirements/2.cc: Update the
	test with P3247R2's new wording.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>