Remove direct use of pgtype.Array with pgx/v5

[earthboundkid]

Fixes #2083

[kyleconroy]

I don't know the context for the sqlc thing, but I'm a little surprised whenever I see pgtype.FlatArray or especially pgtype.Array used directly. Typically, pgx can directly work with []T, [][]T, etc. I only anticipated pgtype.Array being used directly if the application actually needed to work with non-standard starting indexes.

What about nullable values? If I have a three strings and a NULL, I don't think those can be scanned into a []string.