dotnetup update should help you understand what updates are available but not applied

If you specify a specific version of the SDK or runtime when you install, then you'll be pinned to that version and it won't be updated. That may not be what everyone intends.

So when you run `dotnetup update`, we should probably display messages if there are newer patches available for versions that are pinned, or if there are versions installed that are out of support and won't receive updates. This might be similar to the information that the `dotnet sdk check` command provides.

We might also want to print a message when a specific version to install is specified. See https://github.com/dotnet/sdk/discussions/55173