Modernize hosting-model code snippets in file-uploads.md to minimal hosting

[tdykstra]

Description

Split out from review of #37600 to keep that PR scoped to its originating UUF issue (#36039).

The Server and app configuration section of aspnetcore/mvc/models/file-uploads.md mixes two hosting-model styles within the same :::moniker range=">= aspnetcore-5.0"::: block:

• Existing snippets use the pre-3.0 style: public void ConfigureServices(IServiceCollection services), CreateHostBuilder / Host.CreateDefaultBuilder(...).ConfigureWebHostDefaults(...), and references to Startup.ConfigureServices.
• Snippets added in Document IISServerOptions.MaxRequestBodySize and IHttpMaxRequestBodySizeFeature in file upload guide #37600 use minimal hosting: builder.Services.Configure<IISServerOptions>(...), app.MapPost(...).

Since this content is scoped to ASP.NET Core 5.0 and later — and the project templates have used minimal hosting since 6.0 — the older snippets should be updated so the section reads consistently.

Snippets to update

All in aspnetcore/mvc/models/file-uploads.md, within the >= aspnetcore-5.0 moniker block:

• Multipart body length limit — services.Configure<FormOptions> wrapped in public void ConfigureServices(...)
• Multipart body length limit — services.AddRazorPages(options => ...) convention example; prose references Startup.ConfigureServices
• Kestrel maximum request body size — CreateHostBuilder / ConfigureWebHostDefaults / webBuilder.ConfigureKestrel / .UseStartup<Startup>()
• Kestrel maximum request body size — services.AddRazorPages(options => ...) RequestSizeLimitAttribute convention example; prose references Startup.ConfigureServices

Suggested target style, matching the rest of the article after #37600:

var builder = WebApplication.CreateBuilder(args);

builder.Services.Configure<FormOptions>(options =>
{
    // Set the limit to 256 MB
    options.MultipartBodyLengthLimit = 268435456;
});

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.ConfigureKestrel(options =>
{
    // Handle requests up to 50 MB
    options.Limits.MaxRequestBodySize = 52428800;
});

Notes

• Update the surrounding prose that references Startup.ConfigureServices so it matches the minimal hosting snippets.
• Verify whether any of these need to remain in the older style for the >= aspnetcore-3.0 < aspnetcore-5.0 and 2.1 moniker blocks — those blocks are separate and out of scope here.
• Bump ms.date when making the change.