[wwwdocs, pushed] Document libstdc++ headers that are deprecated in GCC 15

[Jonathan Wakely]

On Sun, 16 Feb 2025 at 18:42, Gerald Pfeifer <gerald@pfeifer.com> wrote:
>
> On Thu, 16 Jan 2025, Jonathan Wakely wrote:
> > Pushed to wwwdocs.
>
> Thank you for updating the release notes!
>
> > +<h3 id="deprecated-headers">Deprecated headers</h3>
> > +<p>
> > +Some C++ Standard Library headers now produce deprecation warnings when
> > +included. The warnings suggest how to adjust the code to avoid the warning,
> > +for example all uses of <code><cstdbool></code> and
> > +<code><cstdalign></code> can simply be removed,
> > +because they serve no purpose and are unnecessary in C++ programs.
> > +Most uses of <code><ccomplex></code> can be adjusted to use
> > +<code><complex></code> instead, and uses of <code><ctgmath></code>
> > +can use <code><cmath></code> and/or <code><complex></code>.
> > +</p>
>
> May I suggest to keep issuing these deprecation warnings, alas not remove
> these headers with GCC 16?

I don't think we've ever actually removed something that was in the
standard and then deprecated and eventually removed. We still support
std::auto_ptr with -std=c++26 and that was removed for C++11.

> There likely is just too much software that would break.