Consider targeting all supported .NET versions in Microsoft.AspNetCore.OpenApi

### 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.

Today, Microsoft.AspNetCore.OpenApi NuGet package of a specific release targets the same corresponding .NET version.

For example, Microsoft.AspNetCore.OpenApi 10.x targets net10.0.

This introduces some challenges:

- New bug fixes has to be evaluated for servicing older versions, and can be rejected if the fix is big enough or feels risky to backport.
- New breaking-change (major version) releases of OpenAPI.NET are problematic.
- Consider a user using .NET 10 today, relying on Microsoft.OpenApi 2.x
- User wants to use Kiota in the same project. The latest version of Kiota supports Microsoft.OpenApi 3.x.
- Both can't work together. The user is effectively locked on OpenApi 2.x.

### Describe the solution you'd like

If Microsoft.AspNetCore.OpenApi 11.x targets net8.0, then users on net8.0 through net10.0 can consume the 11.x version of the package, even if they are not fully on .NET 11.

This allows users to get bug fixes easier, and also allows them to consume the latest OpenAPI.NET package.

I'm not sure also if this could simplify the support matrix for OpenAPI.NET itself? @baywet

### Additional context

See related issue https://github.com/microsoft/kiota/issues/7966

The issue names Microsoft.AspNetCore.OpenApi and the related Kiota issue, but no source files or tests. Start by locating the package's target-framework configuration and reviewing the supported .NET and OpenAPI.NET version matrix; done means an agreed targeting and support plan.

Written by the indexing model from the issue text.