Remove support for .NET 6

[amerjusupovic]

The .NET 6 release is no longer supported.

[zhiyuanliang-ms]

Any updates on droping .net 6?

[zhiyuanliang-ms]

We should also update AzureAppConfigurationRefresherProvider and AzureAppConfigurationHealthCheck. See here in the future.

ChainedConfigurationProvider.Configuration is public since .NET 8+

[amerjusupovic]

@zhiyuanliang-ms Thanks for the reminder on things we need to update. We always remove support for .NET versions in a major release, so we'll have to consider when version 9.0.0 of the provider will be released.

[zhiyuanliang-ms]

#695