Update Program.cs example for minimal-apis documentation

[Huntk23]

Utilize Microsoft.AspNetCore.Http.StatusCodes instead of integers for Produces() method of a endpoint mapping to utilize available convention within the AspNetCore framework.

Fixes #24360

[Rick-Anderson]

@Huntk23 did you test the project compiles? Does it need an include?

[Huntk23]

Thankfully, no includes are needed. When I tested, the StatusCodes- they fall under Microsoft.AspNetCore.App > Microsoft.AspNetCore.Http which are included in using a .NET 6.0+ project SDK of Microsoft.NET.Sdk.Web (this also includes the new minimal-API WEB API template packed with .NET 6.0+)

Confirmed that cp2do.csproj is correctly up-to-date so we don't need any includes/usings.

Edit: To further clarify; yes, Program.cs compiles

[Rick-Anderson]

Much appreciated. Nice improvement.
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