NuGet updater should try to fix `NU1605` package downgrade errors

The NuGet updater attempts to fix `NU1608` package inconsistency errors, but doesn't handle `NU1605` package downgrade errors. The fix is likely the same or very similar for both.

Locate the existing handling for NU1608 package inconsistency errors and trace how the NuGet updater decides whether it can fix them. Compare that path with NU1605 downgrade errors and verify the updater handles the downgrade case consistently, including coverage for the reported error.

Written by the indexing model from the issue text.