Bugfix: Warn when upcast drops nullness via FindUniqueFeasibleSupertype

[T-Gro]

Fixse #18232

Repro which does not warn, but should:

module Test

open System.IO
open System

// This is bad - input is nullable, output is not = must warn
let whatisThis (s:Stream|null) : IDisposable =
    s

[github-actions]

❗ Release notes required

@T-Gro,

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