We already supported this feature, but couldn't set the feature test
macro accordingly because we were missing support for older features.
Now that we support all the older <format> changes, we can set this to
the correct value.

libstdc++-v3/ChangeLog:

	* include/bits/version.def (format): Update value for C++26.
	* include/bits/version.h: Regenerate.
	* include/std/format (runtime_format, wruntime_format): Check
	__cpp_lib_format instead of __cplusplus.
	* testsuite/std/format/functions/format.cc: Update expected
	value of macro for C++26 mode.