Cease making win32 builds for S3_1 and later
I don't think anybody needs 32bit Windows builds any more, especially by the time 3.1.0 gets released. Save the resources by ceasing to build win32 builds from S3_1 and master.
As it's likely that some msys2 environment will be backported to use in S3_0, we can't drop support for win32 from them yet. We can do that once the new msys2 environment versions will be used for S3_1+ only.
As part of #43200, this has also been posted to http://forum.freeciv.org/f/viewtopic.php?f=14&t=92047