Qt6Base_jll on older Julia versions on Windows

[barche]

In the upgrade to Qt 6.8 I moved to GCC 13 for the Windows builds to avoid an internal compiler error. As it turns out, this makes the binaries incompatible with anything older than Julia 1.10 due to bundled libstdc++. So now I see 3 ways around this:

1. Do nothing and explain where the problem comes from if anyone on Windows uses this with older versions of Julia
2. Bump the Julia compat on the next version bump, although this is rougher than needed since Windows is the only platform affected by the problem
3. Attempt to fix the internal compiler error in a way that still allows to use an older GCC version. Note that this goes against Qt's own supported compiler version recommendation: https://doc.qt.io/qt-6.8/supported-platforms.html

I prefer option 1 myself, but I just wanted everyone to be aware of this, since right now just installing and using Plots.jl with default settings doesn't work on e.g. Julia 1.6 on Windows (because GR uses the new Qt).

The exact error you get will be "The specified procedure could not be found.": https://github.com/JuliaGraphics/QML.jl/actions/runs/15409893577/job/43359579903#step:5:271

Same error happens on the GR.jl CI: https://github.com/jheinen/GR.jl/actions/runs/15208586946/job/42789512264

CC @jheinen

[jheinen]

I prefer option 1 myself, but I just wanted everyone to be aware of this, since right now just installing and using Plots.jl with default settings doesn't work on e.g. Julia 1.6 on Windows (because GR uses the new Qt).

I would also prefer option 1 - also because GCC-10 is no longer supported.

[jheinen]

Since the GCC-10 problems only occur on Windows 32 bit systems, we could also decide to discontinue i686 support only.

[ViralBShah]

I think option 1 is best.