Update .NET version to .NET 8 preview

[Scott-Caldwell]

Update to .NET 8 preview

• Switch to System.Text.Json
• Use System.Text.Json source generators
• Use regex source generators
• Use SqlQuery instead of FromSql
• Always use UTC time
• Use generic host
• Remove dead code

Tested everything, and I don't see any regressions.

[Scott-Caldwell]

No clue why it's complaining about npm. Going to close for now and try to isolate the issue.