go-tuf improperly validates the configured threshold for delegations

go-tuf is a Go implementation of The Update Framework (TUF). Starting in version 2.0.0 and prior to version 2.3.1, a compromised or misconfigured TUF repository can have the configured value of signature thresholds set to 0, which effectively disables signature verification. This can lead to unauthorized modification to TUF metadata files is possible at rest, or during transit as no integrity checks are made. Version 2.3.1 fixes the issue. As a workaround, always make sure that the TUF metadata roles are configured with a threshold of at least 1.