Update SDK to .NET 7

[adamsitnik]

We have a test that ensure that latest NativeAOT is supported:

BenchmarkDotNet/tests/BenchmarkDotNet.IntegrationTests/NativeAotTests.cs

Line 18 in c165ba1

public void LatestNativeAotVersionIsSupported()

Recently it got broken because we can't build a net6.0 app using .NET 7 NativeAOT bits:

 Unable to optimize assemblies for Ahead of time compilation: a valid runti
me package was not found. Either set the PublishAot property to false, or use a supported runtime identifier when publishing. When targeting .NET 7 or higher, make sure to restore packages with the PublishAot property set to true.

So as soon as .NET 7 SDK is shipped, we should just update our CI to use it.

cc @MichalStrehovsky

[adamsitnik]

At the time of doing this, we should remove the support for old experimental packages (from the old CoreRT https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-experimental/nuget/v3/index.json feed)

[timcassell]

Looks like this is already done since 61b3c56, right @adamsitnik?

[adamsitnik]

@timcassell that is correct, thanks for catching it!

fixed with #2209 in 0.13.3