Drop ciso646 deprecated header

[jschueller]

Else I get a warning with gcc 15:

/usr/include/c++/15/ciso646:46:4: warning: #warning "<ciso646> is deprecated in C++17, use <version> to detect implementation-specific macros" [-Wcpp]
   46 | #  warning "<ciso646> is deprecated in C++17, use <version> to detect implementation-specific macros"

I checked including vector instead also yields the _LIBCPP_VERSION macro with clang/libc++

Also fixes the doc CI job

[jschueller]

/cc @bluescarni

[bluescarni]

Thanks @jschueller !

The idea with ciso646 was to include the lightest possible header that would allow us to detect the standard library version. Had no idea this was being deprecated...