RemoveOneToManyDeleteConvention.cs

// ASP.NET Framework
modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();

// ASP.NET Core
foreach (var entityType in modelBuilder.Model.GetEntityTypes())
{
    foreach (var navigation in entityType.GetNavigations())
    {
        navigation.ForeignKey.DeleteBehavior = DeleteBehavior.Restrict;
    }
}