preferences: Fix error after changes to Plugins section

The Plugins section in the preferences dialog allows to enable/disable
plugins. After making changes and pressing OK or Apply in the
preferences dialog, an error occurred.

The error occurs because of a change of the return type of
SpyderConfigPage.create_checkbox() (see PR20926 [1]).

[1] spyder-ide#20926

spyder/api/plugin_registration/_confpage.py

@@ -57,7 +57,7 @@ def setup_page(self):
                 )
             )
 
-            self.plugins_checkboxes[plugin_name] = (cb, plugin_state)
+            self.plugins_checkboxes[plugin_name] = (cb.checkbox, plugin_state)
 
         # ------------------ External plugins ---------------------------------
         for plugin_name in self.plugin.all_external_plugins:
@@ -86,7 +86,7 @@ def setup_page(self):
                 )
             )
 
-            self.plugins_checkboxes[plugin_name] = (cb, plugin_state)
+            self.plugins_checkboxes[plugin_name] = (cb.checkbox, plugin_state)
 
         # Sort elements by title for easy searching
         collator = Collator()