[libc++] Bump the C++ Standard used to compile the dylib to C++23 (#66824)

This is necessary in order to implement some papers like P2467R1, which
require using C++23 declarations in the dylib. It is a good habit to
keep building the dylib with a recent standard version regardless.

With this patch, we also stop strictly enforcing that the targets are
built with C++23. Concretely, C++23 will soon be required in order to
build the dylib, but not enforcing it strictly works around some issues
like the documentation bots using an old and unsupported compiler. Since
these bots do not actually build the library, not strictly enforcing the
C++ Standard makes our CMake build more resilient to these kinds of
situation. This is just a workaround though, the better way of going
about would be to update the compiler on the documentation bot but we
don't seem to have control over that.

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