Document HostingStartup incompatibility and migration to modern hosting patterns for .NET Aspire

[Copilot]

This PR addresses the incompatibility between HostingStartup and .NET Aspire integrations by providing comprehensive migration guidance from the legacy hosting pattern to the modern IHostApplicationBuilder approach.

The Problem

.NET Aspire integrations require IHostApplicationBuilder, but HostingStartup only provides access to the older IWebHostBuilder. This fundamental API difference means:

• Aspire integration methods like AddNpgsqlDbContext() are not available
• Modern hosting features (service discovery, built-in telemetry, health checks) can't be configured
• Applications using HostingStartup miss out on .NET Aspire's observability and resilience patterns

The Solution

The documentation emphasizes migration as the necessary path forward rather than workarounds, recognizing that HostingStartup represents decade-old technology that predates modern .NET hosting patterns.

Key Content

• Clear explanation of why the incompatibility exists at the API level
• Before/after examples showing the transition from IWebHostBuilder to IHostApplicationBuilder
• Conceptual guidance on the hosting model changes introduced in ASP.NET Core 6.0+
• Migration resources linking to official ASP.NET Core migration guides and David Fowl's migration patterns

Example Migration

Before (HostingStartup - incompatible):

public class MyDatabaseStartup : IHostingStartup
{
    public void Configure(IWebHostBuilder builder) // ❌ Wrong builder type
    {
        builder.ConfigureServices(services => { ... });
    }
}

After (Modern hosting - compatible):

var builder = WebApplication.CreateBuilder(args);
builder.AddServiceDefaults();                    // ✅ Aspire integration
builder.AddNpgsqlDbContext<MyDbContext>("postgres"); // ✅ Works correctly

This approach guides developers toward modern .NET patterns while ensuring they can leverage .NET Aspire's full feature set including service discovery, telemetry, and resilience capabilities.

Fixes #3861.

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/integrations-overview.md	docs/fundamentals/integrations-overview
docs/toc.yml	docs/toc
docs/troubleshooting/hosting-startup-not-supported.md	docs/troubleshooting/hosting-startup-not-supported

[IEvangelist]

@copilot - Please remove option 4 and the example source code for it.

[Copilot]

Removed Option 4 and the dependency injection plugin example code as requested. Fixed in commit 7b9a6ef.

[IEvangelist]

@copilot - I don't want multiple options listed. Instead, the only option is to migrate. It's time to force their hand so to speak, but really, we're talking about technology that's fast approaching (if not already) over a decade old. When presenting options, push the migration but we don't need to spell it out for them since we already have content for that. Instead, talk through the concepts and the changes to the APIs. Reference things like:

• https://learn.microsoft.com/aspnet/core/migration/50-to-60?view=aspnetcore-9.0
• https://gist.github.com/davidfowl/0e0372c3c1d895c3ce195ba983b1e03d

[Copilot]

Restructured the documentation to focus on migration as the only path forward rather than presenting multiple options. The content now emphasizes the conceptual differences between the older HostingStartup pattern and the modern IHostApplicationBuilder pattern, and references the official ASP.NET Core migration guides and David Fowl's migration resources for detailed guidance. Removed the multiple "options" sections and kept only the essential before/after example to illustrate the API changes. Fixed in commit 329b9fc.