Msys2: Make Qt6-client builds for "win64-10" only
As Qt6 requires Windows 10, there's not much point to build otherwise win < 10 compatible "win64" versions of Qt6-client & ruledit. Have them for "win64-10" only.
As Qt6 requires Windows 10, there's not much point to build otherwise win < 10 compatible "win64" versions of Qt6-client & ruledit. Have them for "win64-10" only.