Consolidate spo site add and spo site classic add commands #1493

Related to #1491

Consolidate spo site add and spo site classic add commands into spo site add which can create both modern and classic sites. Additionally, mark the spo site classic add command as deprecated so that we can remove it in v3.

Once this item is done, let's create a new item linked to milestone v3 to remove the deprecated spo site classic add command.