Support .NET 8 Targeting framework in Microsoft.FeatureManagement.Telemetry.ApplicationInsights.AspNetCore

[zhenlan]

Looks like we missed net8.0 support in the Microsoft.FeatureManagement.Telemetry.ApplicationInsights.AspNetCore packages

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