Drop support for EOL versions of WSL

[K900]

The Windows Store version of WSL is available for everyone running Windows 10 21H1 or up.

Windows 10 21H1 is EOL since December.

We should remove support for those versions, which will dramatically simplify maintenance.

Things that can be removed once we only support Store WSL:

• the installer can be removed, tarball generation substantially rewritten
  • the purpose of the installer was to workaround a libarchive bug with wsl --import which is resolved on Store
  • being addressed in Enable direct installation with native systemd #243
• Docker native workarounds can be fully removed, the latest Store kernel supports nftables
• interpreter magic can be removed in favor of a consistent registration
• old tmpfiles workarounds can be dropped (probably more?)
• non-native-systemd code can be dropped
  • all
  • of
  • it
  • gone
• maybe more?

[K900]

Also, pretty much any of those items can be done independently, so PRs are appreciated.

[K900]

Mostly done for 23.05, still need to kill syschdemd.