Status code :unprocessable_entity is deprecated and will be removed in a future version of Rack

[tagliala]

Status code :unprocessable_entity is deprecated and will be removed in a future version of Rack. Please use :unprocessable_content instead.

Refs:

• https://github.com/activeadmin/activeadmin/actions/runs/17497929587/job/49703441332?pr=8802
• Update HTTP status codes and associated symbols rack/rack#2137

[tagliala]

Hopefully this is not needed because of rails/rails#53383, and the user can always override in an initializer