Include 'marketing' `net8.0` targets for Ix.NET?

Currently, we're planning for certain Ix.NET v7 components to have `net10.0` targets because they need those to be different from earlier .NET versions, but not to offer a `net8.0` target. The existence of a .NET standard target means that the components will run on .NET 8 (and all the way back to .NET Core 3.1) without needing any `netX.0` targets.

However, history shows that people get confused by this, and think that this means the components aren't supported on .NET 8.0.

We will support Ix.NET v8 on .NET 8.0, .NET 9.0 and .NET 10.0. So we need to think about whether to include a technically unnecessary `net8.0` target just so people don't get confused.

This won't be appropriate for `System.Linq.Async` because that's being deprecated anyway (in favour of the .NET runtime libraries' `System.Linq.AsyncEnumerable`) but for all the other Ix components, we need to consider this.

We also need to work out whether any of the components do in fact do something different for their `net6.0` target today, in which case, we might need a `net8.0` target for technical reasons.