Offline Cache API deprecated

We recently revisited the problems we were trying to solve with offline caching since this library was launched in 2019. We found that OfflineCache was being used as a fallback configuration source in case the App Configuration service is down. However, storing configuration data in a local environment outside your App Configuration store can potentially lead to issues. These include difficulties enforcing data access policies and retaining sensitive information in unreliable environments. In the last two years since we went to GA, we have invested heavily in server- side resiliency and availability. Please check our [recommendation for highly available](https://docs.microsoft.com/azure/azure-app-configuration/concept-disaster-recovery#high-availability-architecture) App Configuration.

Therefore, we are deprecating all offline caching capabilities with the release of [v5.0.0 preview](https://github.com/Azure/AppConfiguration/blob/main/releaseNotes/MicrosoftExtensionsConfigurationAzureAppConfiguration.md), in favor of improving server-side reliability.

As always, please feel free to [let us know](https://github.com/Azure/AppConfiguration/issues/new) if you have any feedback.