DE0006: Non-generic collections shouldn't be used

Motivation

When .NET was created, generic data types didn't exist, which is why the
collection types in the [System.Collections][collections] namespace are untyped. However, since then,
generic data types were introduced and thus a new set of collections
were made available in the [System.Collections.Generic][generic] and
[System.Collections.ObjectModel][objectmodel] namespaces.

Recommendation

For new code, you shouldn't use non-generic collections:

• Error prone: since non-generic collections are untyped, it requires frequent
  casting between object and the actual type you're expecting. Since the compiler
  can't check that your types are consistent, it's easier to put the wrong type in
  the wrong collection.

• Less performant: generic collections have the advantage that value types
  don't have to be boxed as object. For instance, a List<int> stores its data
  in an int[]. That's far better than storing the data in object[] as that
  requires boxing.

The following table shows how the non-generic collection types can be
replaced by their generic counterparts from the [System.Collections.Generic][generic] or
[System.Collections.ObjectModel][objectmodel] namespaces:

https://github.com/dotnet/platform-compat/blob/master/docs/DE0006.md