Fixes #2262 and #2263.

This PR:

• Bumps the .NET SDK to 9.0.201.
• Removes the .NET 6.0 TFMs, since .NET 6.0 is no longer supported.
• Replaces all the ASP.NET Core 2.1 references to use the new 2.3 packages released mid-January for the .NET Framework and .NET Standard TFMs, including the .NET Standard 2.1 TFM that previously referenced ASP.NET Core 3.1 packages (which are no longer supported).
• Replaces the Entity Framework Core 2.1 references to use the new 2.3 packages that shipped as part of the 2.1 -> 2.3 change for the .NET Framework and .NET Standard TFMs, including the .NET Standard 2.1 TFM that previously referenced Entity Framework Core 3.1 package (which is no longer supported).
• Replaces the .NET Extensions 2.1 references to use the 8.0 version for the .NET Framework and .NET Standard TFMs, including the .NET Standard 2.1 TFM that previously referenced the 3.1 version. This matches the new pattern used by the ASP.NET Core 2.3 packages that all use .NET Extensions 8.0 instead of 2.1.
• Updates the .NET Framework and .NET Standard TFMs to support TimeProvider. As part of this change, the TimeProvider options are no longer nullable.
• Updates the .NET Framework and .NET Standard TFMs to support System.Text.Json.Nodes.
• Bumps all the .NET and third-party dependencies used for the .NET Framework and .NET Standard TFMs, including IdentityModel.
• Updates the console sandbox to use Host.CreateApplicationBuilder().
• Updates the binding redirects of the ASP.NET samples, which is required due to the .NET Extensions 2.1 -> 8.0 change.
• Updates the web providers generator to implement IIncrementalGenerator instead of ISourceGenerator, as the latter is now deprecated/discouraged in the Roslyn version now referenced by OpenIddict.
• Removes all the #if guards that are no longer necessary.
• Drastically simplifies some of the conditions used in the .targets, .csproj and .props files.