🐛 Allow redownloads on macOS 12 Monterey

[chris-araman]

This change:

• disables the account command on macOS 12
  • 🐛 mas account doesn't work on macOS 12 and later #417
• disables the purchase command on macOS 10.15
  • 🐛 [BUG] Purchasing doesn't work on macOS 10.15 and later #289
• documents why some commands are disabled on some macOS versions
  • 🚏📥🙅‍♀️ Signin command disabled on macOS 10.13+ #164
• documents why the App Store app and mas don't always agree on which apps are outdated, and...
  • 🐛 [BUG] some apps are always shown as outdated even though they're already the latest version #384
  • 🐛 [BUG] App Store GUI sometimes reports apps installed by mas as outdated but mas doesn't #387
• 🥁 fixes install, lucky, and (I believe 🤞🏼) upgrade for existing purchases on macOS 12 🥳
• removes the warning added in 1.8.4 about functionality on macOS 12

[phatblat]

Great idea of grouping the known issues on the readme. This makes me sad how things keep breaking but such is the nature with private APIs 😢