msys2-32bit CI job fails with "error: target not found: mingw-w64-i686-dtc"

Our msys2-32bit CI job is failing again, because upstream MSYS2 have removed 32-bit support from another of the packages we depend on. 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.

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

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