Target netstandard2.* in Microsoft.FeatureManagement.Telemetry.ApplicationInsights

[zhenlan]

The Microsoft.FeatureManagement.Telemetry.ApplicationInsights package targets net6.0 today, but it should just target netstandard2.*. All its dependencies target netstandard.

• https://github.com/microsoft/FeatureManagement-Dotnet/blob/preview/src/Microsoft.FeatureManagement.Telemetry.ApplicationInsights/Microsoft.FeatureManagement.Telemetry.ApplicationInsights.csproj#L15