Sunsetting Pext #527

It's time to admit it: Pext is dead. It has been barely maintained for years, there are some pretty nasty bugs unfixable without major rewrites, the codebase is no longer maintainable and it definitely won't be upgraded to Qt6.

Sunsetting plan:

1. Archive the website (Wayback machine and archive.is)
2. Ensure https://pext.io/download/ will instead of download instructions contain information about Pext being unmaintained and urge any existing users (if any) to uninstall and find a replacement.
3. Create a fake high version in /version/stable and /version/nightly to cause existing installs to alert users about a version update and bring them to https://pext.io/download/
4. Archive the website one last time
5. Set all the GitHub repositories to archived
6. Let the domain expire

All of the changes will need to be made in https://github.com/Pext/Website.

Thank you to everyone who helped with the project over the years. If anyone is still around, thank you, really.