Fix UniqueUsernameValidator comparison (#6926)

Comparison was downcasing only one side, therefore if previously
existing account had a non-lowercase spelling, it would be ignored
when checking for duplicates.

New rake task `mastodon:maintenance:find_duplicate_usernames` will
help find constraint violations that might have occured from the
presence of this bug.

Bump version to 2.3.3

app/models/concerns/account_finder_concern.rb

@@ -30,7 +30,7 @@ module AccountFinderConcern
     end
 
     def account
-      scoped_accounts.take
+      scoped_accounts.order(id: :asc).take
     end
 
     private

app/validators/unique_username_validator.rb

@@ -6,7 +6,7 @@ class UniqueUsernameValidator < ActiveModel::Validator
 
     normalized_username = account.username.downcase.delete('.')
 
-    scope = Account.where(domain: nil, username: normalized_username)
+    scope = Account.where(domain: nil).where('lower(username) = ?', normalized_username)
     scope = scope.where.not(id: account.id) if account.persisted?
 
     account.errors.add(:username, :taken) if scope.exists?