Deprecate mechanical

Posted under Tags

The bulk update request #11842 has been approved by @evazion.

deprecate mechanical

Ambiguous tag.

Also came across mechanical bull, mechanical horse, and mechanical bird while making the disambiguation wiki for mechanical. Should these tags be aliased to robot_* for consistency with robot dog and robot_animal, or just merged into robot animal altogether?
There's also mechanical dragon but I'm not sure if that one should also get lumped into mechanical animal as well.