Update to .NET 8 preview

[Scott-Caldwell]

Update to .NET 8 preview

• Use UTC time
• Use SqlQuery instead of FromSql
• Use generic host
• Remove dead code
• Add missing nullable annotations

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

Additional notes:

• Build failed on previous PR because it was pulling a newer version of nodejs from a different source than we were using, so I've updated the Dockerfile to pin the preferred nodejs source.
• Switched all code to use UTC, because npgsql is now enforcing timestamp with timezone columns to use UTC time. (These columns were always UTC in the database regardless.)
• Using SqlQuery instead of FromSql allows us to stop pretending that certain models are entities, making for cleaner, easier to maintain code.
• Switched IReadOnlyCollection to T[] in search criteria models because npgsql is currently running into errors translating non-arrays into SQL.