libstdc++: Disable Doxygen GROUP_NESTED_COMPOUNDS config option

libstdc++: Disable Doxygen GROUP_NESTED_COMPOUNDS config option

Before Doxygen version 1.9.2 this option is broken (see
https://github.com/doxygen/doxygen/issues/8638 for more details) and
classes are not added to the correct groups by @ingroup and @addtogroup.

Also remove the obsolete CLASS_DIAGRAMS option that causes a warning.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (GROUP_NESTED_COMPOUNDS): Set to NO.
	(CLASS_DIAGRAMS): Remove obsolete option.

(cherry picked from commit 9c3a8fe34aeacabc2f62acef7f8f9108cb109238)