Dropping support for Windows Vista, 7, 8 & 8.1

[erengy]

• Windows 7 reached end of life on January 14, 2020. It no longer receives security updates or fixes. Windows 8.1 is on extended support until January 10, 2023.

• Steam's hardware survey says Windows 7, 8 & 8.1 users make up ~4.24% in total. When we dropped support for XP (Consider ending Windows XP support #91), its share was ~3.87% in the same survey.

  OS Version	Percentage
  Windows 10 64-bit	81.74%
  Windows 11 64-bit	10.15%
  Windows 7 64-bit	3.24%
  Windows 8.1 64-bit	0.68%
  Windows 7	0.19%
  Windows 10	0.08%
  Windows 8 64-bit	0.05%

• If we decide to switch to Qt 6 or WinUI 3, we have to do this since they both require Windows 10 or above.

This is not an immediate change. Taiga's next release (v1.4), at least, will continue working on these older versions of Windows. But I'm planning to drop the support whenever the need arises.