libstdc++: Define C++26 member visit for std::basic_format_arg [PR110356]

Implement the std::format changes from P2637R3. This adds visit member
functions to std::basic_format_arg and deprecates the non-member
function std::visit_format_arg.

libstdc++-v3/ChangeLog:

	PR libstdc++/110356
	* include/bits/c++config (_GLIBCXX26_DEPRECATED): Define.
	(_GLIBCXX26_DEPRECATED_SUGGEST): Define.
	* include/bits/version.def (format): Update for C++26.
	* include/bits/version.h: Regenerate.
	* include/std/format (basic_format_arg::visit): New member
	functions.
	(visit_format_arg): Add deprecated attribute.
	* testsuite/std/format/arguments/args.cc: Expect deprecated
	warnings. Check member visit.
	* testsuite/std/format/functions/format.cc: Update expected
	value for __cpp_lib_format macro.
	* testsuite/std/format/parse_ctx.cc: Add dg-warning for
	deprecation.