[Proposal]: `Task<T>` nullability covariance

# `Task` nullability covariance

* Specification: None yet
* Discussion: https://github.com/dotnet/csharplang/discussions/8996

## Summary
[summary]: #summary

`Task` nullability covariance (LDM tentatively approved, needs design proposal for task-like types)

It should be possible to return a `Task` for a `Task`.

```cs
using System.Threading.Tasks;
#nullable enable
public class C
{
public Task<(bool Result, string? Extras)> M()
{
return Task.FromResult((true, "")); // currently produces a warning
}
}
```

## Design meetings

Split issue from https://github.com/dotnet/csharplang/issues/3868
https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-28.md#nullability-improvements
https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-06.md#taskt-nullability-covariance