Fix unprocessable entity deprecation warning

I'm seeing this deprecation when using newer versions of `rack` with
request specs that use the `rspec-rails` `have_http_status` matcher [1].

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

The `:unprocessable_entity` symbol was deprecated in `rack` in this
commit [2] which was released in `rack` v3.1.0 [3].

[1]: https://rspec.info/features/8-0/rspec-rails/matchers/have-http-status-matcher/
[2]: rack/rack#2137
[3]: https://github.com/rack/rack/blob/ee7ac5a1db5bc5c65e4b83342b8f4df88ef3c075/CHANGELOG.md#310---2024-06-11

Author: floehopper

app/controllers/administrate/application_controller.rb

@@ -54,7 +54,7 @@ def create
       else
         render :new, locals: {
           page: Administrate::Page::Form.new(dashboard, resource)
-        }, status: :unprocessable_entity
+        }, status: :unprocessable_content
       end
     end
 
@@ -68,7 +68,7 @@ def update
       else
         render :edit, locals: {
           page: Administrate::Page::Form.new(dashboard, requested_resource)
-        }, status: :unprocessable_entity
+        }, status: :unprocessable_content
       end
     end