`aspire new` starter - what is `mtp` in xunit `v3mtp`

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

Try to create a starter template with xunit tests, and you get to the following option

I don't think the average dotnet dev (let alone a polyglot dev) would know what `mtp` means, and thsu what the difference between `v3` and `v3mtp` is.

### Describe the solution you'd like

At the very least, I think the `mtp` acronym should be expanded to the full name `microsoft-test-platform`, which may give a user more inform

That said, I lean more towards being opinionated and picking the test framework for V3, either by
- v2 is vstest based, whilst v3 always uses mtp.
- removing the MTP option

### Additional context

Consistency wise, there is no `mtp` option for either nunit or mstest. All three frameworks support mtp now, so I'd expect this to be handled somewhat consistently between the test frameworks.

The issue concerns the `aspire new` starter-template prompt and its xUnit `v3` and `v3mtp` options; start by locating that prompt and comparing how NUnit and MSTest options are defined. Done means the options consistently explain or handle Microsoft Test Platform support, with the chosen behavior reflected in the starter experience.

Written by the indexing model from the issue text.