[Samples] Use ASP.NET Core 6 scaffoldable template as a base of Server and Client samples

[xperiandri]

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Describe the solution you'd like

I propose:

1. Remove your custom AccountController and ManageController, all related views and view models, and rely on Identity Razor pages
2. Make template scaffoldable, so that consumers can scaffold any built-in Identity Razor Page
3. Remove all legacy stuff like gulp, package.json, etc., and use everything from .NET 6 template. Leaving only regular Program.cs and Startup.cs

My company can prepare a PR if you approve

Additional context

As a preparation to project template creation

[kevinchalet]

It's important to note that the "samples" contained in this repo are not really "public samples", unlike the projects you can find in the dedicated repo: https://github.com/openiddict/openiddict-samples.

These projects are actually engineering samples I use when developing/testing new features. Think of them as functional/integration tests projects (that's also the reason why I recently moved them from samples to sandbox and renamed them).

As such, I'm not sure it's worth spending time on 1/2: there's no real ROI to expect here 😄 Regarding 3, I'm not opposed to remove these artifacts, so 👍🏻