Feat/support net 9

[Dovchik]

Remove support of .net7 and .net6 according to
https://dotnet.microsoft.com/en-us/download/dotnet

[asein-sinch]

It's great to see the end of support for deprecated version.
Before merging the branch to main, can you please also update the readme and prepare a release note on the Sinch community portal?
Your users need to be aware of the drop of support for older versions and what Sinch is offering in term of support in case an issue is found on a previous version of the SDK used by a customer who has not migrated to a supported version of .NET.
This information will also be ingested by the chatbot in order to provide an accurate answer to anybody who asks which .NET versions are supported.

[krogers0607]

It's great to see the end of support for deprecated version. Before merging the branch to main, can you please also update the readme and prepare a release note on the Sinch community portal? Your users need to be aware of the drop of support for older versions and what Sinch is offering in term of support in case an issue is found on a previous version of the SDK used by a customer who has not migrated to a supported version of .NET. This information will also be ingested by the chatbot in order to provide an accurate answer to anybody who asks which .NET versions are supported.

I've approved a community friendly message for this https://community.sinch.com/t5/Developer-Forum/NET-Framework-Updates-for-NET-SDK/m-p/17668#M192. @asein-sinch can you approve this request with this now linked for users?