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/user_search.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class UserSearch
-  MAX_SIZE_PRIORITY_MENTION ||= 500
+  MAX_SIZE_PRIORITY_MENTION = 500
 
   def initialize(term, opts = {})
     @term = term.downcase