Use normalizes to prepare CustomEmoji domain value (#28624)

app/models/custom_emoji.rb

@@ -41,7 +41,7 @@ class CustomEmoji < ApplicationRecord
 
   has_attached_file :image, styles: { static: { format: 'png', convert_options: '-coalesce +profile "!icc,*" +set date:modify +set date:create +set date:timestamp' } }, validate_media_type: false
 
-  before_validation :downcase_domain
+  normalizes :domain, with: ->(domain) { domain.downcase }
 
   validates_attachment :image, content_type: { content_type: IMAGE_MIME_TYPES }, presence: true, size: { less_than: LIMIT }
   validates :shortcode, uniqueness: { scope: :domain }, format: { with: SHORTCODE_ONLY_RE }, length: { minimum: 2 }
@@ -95,8 +95,4 @@ class CustomEmoji < ApplicationRecord
   def remove_entity_cache
     Rails.cache.delete(EntityCache.instance.to_key(:emoji, shortcode, domain))
   end
-
-  def downcase_domain
-    self.domain = domain.downcase unless domain.nil?
-  end
 end