Decide strategy for .NET 9 and Aspire 9 in samples

[DamianEdwards]

.NET 9 and Aspire 9 will be released in November this year. Aspire 9 will support both .NET 8 and .NET 9, i.e. the most recent in-support LTS and STS releases of .NET. Once Aspire 9 is released it will be the only supported version of Aspire as it is a "tip" product with only the latest stable version being supported

We need to decide what to do for the samples in this repo when this happens. They should all be moved to use Aspire 9 as part of the release as that will be the most recent version of Aspire and thus the only supported version. But the question of whether to have the samples continue to target .NET 8, move to .NET 9, or multi-target or be replicated for both, is an open one.

Cc @maddymontaquila