Require .NET Framework 4.8 as the new minimum version

[kevinchalet]

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Describe the solution you'd like

The OpenIddict client, server and validation stacks can be currently used in any .NET Framework 4.6.2+ application (including ASP.NET 4.6.2+ WinForms/MVC/Web API applications).

Microsoft has already announced that 4.6.2 will no longer be supported after January 2027, meaning 4.7 will become the oldest .NET Framework version supported by Microsoft.

Since it will no longer be supported, the net462 target framework moniker will be removed in the next major version, making net472 the oldest version supported by OpenIddict starting with 8.0.

Additional context

This plan matches the strategy adopted by other projects, like xUnit 3.

[kevinchalet]

Open question: would there be objections if we also removed the .NET Framework 4.7.2 TFM and required .NET Framework 4.8 as the minimum version? Both 4.7.2 and 4.8 share the same requirements and can be used on Windows 7 SP1 and Windows Server 2008 R2 SP1.