CBL Xamarin Android : Update min version of supported API

The minimal supported API version on CBL Xamarin SDK is API 19.
Android 9.0 is at API version 29!  So we are currently supporting 10 versions of the API.

API 19 was released almost 6 years ago (2013) and distribution is at at 6.9%

In order to keep the maintenance of the platform more manageable going forward, we will be deprecating support for API 19 starting Cobalt. The exact date when we will remove support is TBD but we want to give opportunity for existing customers to deal with the deprecation.

Task:

Please commit directly on the `release/2.6` branch.

• For cobalt, Please update the minimal API version table to be API 22 (We could set at API 21 but with the issues in 21, API22 is what is adopted).
• Add add a note to indicate that :
  Support for API 19 and API 21 is deprecated.  Support will be removed within two (non-maintenance) releases following the deprecation announcement

 CC Sandy Chuang, Jim Borden, Daniel Petersen