DEV: Change category type to categories type for theme object schema (#26339)

Why this change?

This is a follow-up to 86b2e3aa3e8be30a308f1bff3664d76c5d56057a.

Basically, we want to allow people to select more than 1 category as well.

What does this change do?

1. Change `type: category` to `type: categories` and support `min` and `max`
   validations for `type: categories`.

2. Fix the `<SchemaThemeSetting::Types::Categories>` component to support the
   `min` and `max` validations and switch it to use the `<CategorySelector>` component
   instead of the `<CategoryChooser>` component which only supports selecting one category.

lib/theme_settings_manager/objects.rb

@@ -16,4 +16,21 @@ class ThemeSettingsManager::Objects < ThemeSettingsManager
   def schema
     @opts[:schema]
   end
+
+  def categories(guardian)
+    category_ids = Set.new
+
+    value.each do |theme_setting_object|
+      category_ids.merge(
+        ThemeSettingsObjectValidator.new(
+          schema:,
+          object: theme_setting_object,
+        ).property_values_of_type("categories"),
+      )
+    end
+
+    return [] if category_ids.empty?
+
+    Category.secured(guardian).where(id: category_ids)
+  end
 end