Update target frameworks for .NET 6, .NET Framework 4.7.1, .NET Standard 2.0, .NET Standard 2.1

[Romfos]

What's changed?

• Target frameworks now: .NET 6, .NET Framework 4.7.1, .NET Standard 2.0, .NET Standard 2.1

Why target frameworks were changed?

Microsoft recommend to target .NET 6, .NET Standard and minimal supported .NET Framework https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/6.0/older-framework-versions-dropped

Notes: .NET Framework 4.5 is out of support, .NET Framework 4.6.2 is the older version that is still supported https://devblogs.microsoft.com/dotnet/net-framework-4-5-2-4-6-4-6-1-will-reach-end-of-support-on-april-26-2022/

.NET 5 is out of support https://devblogs.microsoft.com/dotnet/dotnet-5-end-of-support-update/

.NET Core 3.1 is out of support https://devblogs.microsoft.com/dotnet/net-core-3-1-will-reach-end-of-support-on-december-13-2022/

[Romfos]

cc @MichaCo