DEV: Remove logical OR assignment of constants (#29201)

Constants should always be only assigned once. The logical OR assignment
of a constant is a relic of the past before we used zeitwerk for
autoloading and had bugs where a file could be loaded twice resulting in
constant redefinition warnings.

app/models/imap_sync_log.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class ImapSyncLog < ActiveRecord::Base
-  RETAIN_LOGS_DAYS ||= 5
+  RETAIN_LOGS_DAYS = 5
 
   belongs_to :group