`dotnetup` self update should notify `preview` customers of available update

### Prerequisites

- [x] I have read the [dotnetup documentation](https://github.com/dotnet/sdk/tree/release/dnup/documentation/general/dotnetup).
- [x] I have searched for [existing dotnetup issues](https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3AArea-dotnetup).
- [x] I have searched for [existing dotnetup discussions](https://github.com/dotnet/sdk/discussions/categories/dotnetup).

### Issue type

Bug report

### Description

At the end of SDK install or runtime install commands, `preview` dotnetup should check if there's an updated version of `dotnetup` and recommend the customer update with `dotnetup self update` when it is available. This is a stop gap and feedback mechanism to see if folks want `dotnetup` to update itself in these situations instead or if this approach is preferred. Messages might be dropped or removed depending on cadence.

### dotnetup version

_No response_