Deprecate api command

[colemanw]

APIv3 is officially deprecated so civix should reflect that.

Also adds @deprecated annotation to all deprecated commands.

[totten]

Wouldn't it make sense to switch this to the APIv4 equivalent?

One thing - APIv4 is in a funnier position because of how Civi\Api4\MyEntity.php and Civi\Api4\Action\MyEntity\MyAction.php relate. IIRC:

• When you generate an action class, you should update the entity class to declare a stub.
• But only if your extension owns the entity. You skip the stub if entity is owned by someone else.
• Adding the stub function requires modifying the PHP source. That can be fiddly, but there's a big mitigating factor -- you don't have to worry about pre-existing functions. I'd wager 98% cases would work with a little regex to inject the new function at the end of the file (right before the terminal }).