Normalize current_username on account migration (#38183)

app/models/account_migration.rb

@@ -25,7 +25,10 @@ class AccountMigration < ApplicationRecord
   before_validation :set_target_account
   before_validation :set_followers_count
 
+  attribute :current_username, :string
+
   normalizes :acct, with: ->(acct) { acct.strip.delete_prefix('@') }
+  normalizes :current_username, with: ->(value) { value.strip.delete_prefix('@') }
 
   validates :acct, presence: true, domain: { acct: true }
   validate :validate_migration_cooldown
@@ -33,7 +36,7 @@ class AccountMigration < ApplicationRecord
 
   scope :within_cooldown, -> { where(created_at: cooldown_duration_ago..) }
 
-  attr_accessor :current_password, :current_username
+  attr_accessor :current_password
 
   def self.cooldown_duration_ago
     Time.current - COOLDOWN_PERIOD