Backward compatibility with Tableau 2020.1 and older

[rspocz]

The current release of TabMigrate works only with Tableau Server 2020.2 or newer, because it is using REST API 3.8. This behaviour was introduced by change in TabRESTMigrate/RESTHelpers/TableauServerUrls.cs in commit c0afeb8, which uses REST API 3.8 instead of REST API 2.3. Given the commit message, I suspect this change was unintentional. Could you please double check the commit? Is it necessary to use newer version of REST API? Thank you