c++: hide __is_deducible for GCC 13

I want to have more discussion about the interface before claiming the
__is_deducible name, so for GCC 13 make it internal-only.

gcc/ChangeLog:

	* doc/extend.texi: Comment out __is_deducible docs.

gcc/cp/ChangeLog:

	* cp-trait.def (IS_DEDUCIBLE): Add space to name.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_deducible1.C: Guard with
	__has_builtin (__is_deducible).