libstdc++: Implement P2997R1 changes to the indirect invocability concepts

libstdc++: Implement P2997R1 changes to the indirect invocability concepts

This implements the changes of this C++26 paper as a DR against C++20.

In passing this patch removes the std/ranges/version_c++23.cc test which
is now mostly obsolete after the version.def FTM refactoring, and instead
expands the __cpp_lib_ranges checks in another test so that it verifies
the exact value of the FTM on a per language version basis.

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (indirectly_unary_invocable):
	Relax as per P2997R1.
	(indirectly_regular_unary_invocable): Likewise.
	(indirect_unary_predicate): Likewise.
	(indirect_binary_predicate): Likewise.
	(indirect_equivalence_relation): Likewise.
	(indirect_strict_weak_order): Likewise.
	* include/bits/version.def (ranges): Update value for C++26.
	* include/bits/version.h: Regenerate.
	* testsuite/24_iterators/indirect_callable/p2997r1.cc: New test.
	* testsuite/std/ranges/version_c++23.cc: Remove.
	* testsuite/std/ranges/headers/ranges/synopsis.cc: Refine the
	__cpp_lib_ranges checks.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit 620232426bd83a79c81cd2be6f485834c618e920)