Add .NET Standard 2.0 support to Microsoft.AspNetCore.WebUtilities

### 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.

During the 2.x timeframe, `Microsoft.AspNetCore.WebUtilities` was available as a .NET Standard 2.0 NuGet package. It was then moved into the shared framework. However, it is now a package again (as of https://github.com/dotnet/aspnetcore/pull/48265), but only supports .NET 8. People had taken a reliance on it as a package in shared libraries during the migration process but are not able to anymore (they couldn't technically after 2.x, but they just kept the old one it seems).

### Describe the solution you'd like

`Microsoft.AspNetCore.WebUtilities` and `Microsoft.Net.Http.Headers` should support .NET Standard 2.0.

### Additional context

_No response_