Add support for .NET 9.0

[ceciliaavila]

#minor

Description

This PR adds support for .NET 9.0 removing .NET 6.0 as target framework as its end date was on Nov 12, 2024.

Specific Changes

• Updated the projects' target framework removing .NET 6.0 and adding .NET 9.0.
• Updated yaml files in onebranch and yaml folders to remove .NET 6.0 and add .NET 9.0 as targets.
• Configured NETAnalyzers version 9 in Directory.Build.props to avoid warnings at build.
• Set NoWarnings for IL2026 and IL3050 in Microsoft.Bot.AdaptiveExpressions.Core.AOT.Tests project.
• Added a package reference for Microsoft.AspNetCore.TestHost in Microsoft.Bot.Builder.ApplicationInsights.Tests and Microsoft.Bot.ApplicationInsights.Core.Tests projects as it's not part of the SDK in .NET 9.
• Fixed a CA2022 warning by replacing Stream.Read() with Stream.ReadExactly() in Microsoft.Bot.Connector.Tests/AttachmentsTests.
• Updated asserts in 2 tests of TeamsActivityHandlerTests for them to pass in Mac OS pipeline.

Testing

These images show the test bot working after the changes and the CI-pipeline and FunctionalTest pipeline successfully executed.