How the transition should be done

Note: this section should be considered incomplete. I'll appreciate some changes suggested by people who have maintained Qt family in the past.

• Add Qt6 family of packages
• Initial migration of a few packages to link against Qt6
• Manual testing of the packages now linked against Qt6
  • addpkg(x11): qt6-*, bump(x11/feathernotes,featherpad,mkvtoolnix) #19464
• Complete transition to Qt6 for all packages that successfully link against it.
• Deprecate Qt5 family of packages, and finally remove it later