[Proposal] Deprecate .NET 6/7 support in favor of .NET 8 in fall release

[WhitWaldo]

This is more a placeholder for a later release after November of this year when .NET 6 and .NET 7 become officially unsupported. Following that date in the subsequent release, I propose that the .NET package for Dapr be updated to target .NET 8 and that the language version be updated to at least C# 12 so maintainers and developers can utilize the latest language functionality when adding new features (as was a minor issue when implementing the cryptography functionality in which a feature was only available in .NET 8 and an inferior approach was taken because of the .NET 6 dependency).

This will require updating not only the SDK itself, but also examples, quickstarts and potentially a quick pass through the documentation.