Stop building i686 installers/portable Gits/archives

[dscho]

As promised in https://gitforwindows.org/32-bit, Git for Windows provided limited support for i686 until 2025. That year has now arrived.

As per the promise, Git for Windows will still produce the i686 variant of MinGit until April 2029, but that's it.

So what does this entail? For now, I believe that the only change we need to make is to the create-artifacts-matrix.js script, where we need to make the installer, portable and archive artifacts conditional on architecture !== 'i686', similar to how we exclude MinGit-busybox for aarch64.

For continuity, and to give us a chance to communicate this change in the Release Notes of one last Git for Windows version that still has said "limited support for i686", let's make this change just after releasing Git for Windows v2.48.1.

[dscho]

/add relnote blurb Git for Windows v2.48.1 is the last version to ship with the i686 ("32-bit") variant of the installer, portable Git and archive. Only 32-bit MinGit will be built for future versions, until April 2029.

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