Announcement: 2.x is now "done", 3.x work has commenced

[Deadpikle]

On 20 April, 2021, I released version 2.0 of NetSparkleUpdater, which was the culmination of a lot of people's work and a lot of time. Since then, we've done our best to maintain backwards compatibility (see: we're still compatible with .NET 4.5.2!) and use the same libraries and all that...but now it's time to rip stuff out, make some changes, and move this library forward! This is especially so we can make some logistical changes to the way stuff works to fix some long-standing issues.

Back in the day, 2.x was in preview for a really, really long time. I don't want to make 3.x take that long. But, well, time will tell. Life happens. 2.x still works fine, basically, so who knows.

This announcement serves four purposes:

1. If anyone wants to make or suggest any major, breaking changes (read: improvements), now would be a great time to do so. You may do so here or in a separate issue.
2. To let people know that the UPGRADING.md file will have a list of breaking changes in it. If you update to a later preview build and things stop working, check there first.
3. If you choose to use a 3.x preview build, please please please PLEASE test the upgrade process on your end before releasing some software to your users. It's called a preview for a reason. 😉 And please report any bugs you experience! Of course, we hope to not break anything, but, well, that's why we do these sorts of preview things.
4. If we need to backport to 2.x and/or bug-fix to 2.x, we can.

And always remember: contributions are always welcome. 😁 The worst I can do is say "no".

Onward to 3.0!