dropping 32-bit Windows host support

[Peter Maydell]

Our msys2 32-bit Windows host CI job has been failing recently
because upstream MSYS2 are starting to phase out 32-bit windows
host support and are steadily removing i686 versions of packages.
The latest is dtc:
https://gitlab.com/qemu-project/qemu/-/issues/2177

The writing is clearly on the wall for their 32-bit support, judging
from the "2023-12-13 - Starting to drop some 32-bit Packages" news
item at https://www.msys2.org/news/ and associated discussion at
https://github.com/msys2/MINGW-packages/discussions/19326 .

QEMU on a 32-bit host is not likely to be a great experience, and I
suspect we don't have many users using 32-bit Windows who couldn't
use the 64-bit version instead. Our Download page points users at
MSYS2's packages, and they have already dropped the 32-bit QEMU
package build. Stefan Weil's binaries page, which is the other thing
we link to from Download, has no 32-bit version newer than 7.2.0.
So anybody using 32-bit Windows QEMU must be building it themselves.
Plus, we've already deprecated system emulation on 32-bit x86 hosts,
so the only remaining "supported" config is with the NVMM or WHPX
accelerators.

I suggest that we should:

 * remove the msys2-32bit CI job entirely (we will still have at least
   compile-time coverage via the cross-win32-system job)
 * document that the 32-bit Windows support is deprecated in
   docs/about/build-platforms.rst and deprecated.rst
 * update our Download page to remove mention of 32-bit Windows

Any objections?

thanks
-- PMM