DEV: Replace #pluck_first freedom patch with AR #pick in core (#19893)

The #pluck_first freedom patch, first introduced by @danielwaterworth has served us well, and is used widely throughout both core and plugins. It seems to have been a common enough use case that Rails 6 introduced it's own method #pick with the exact same implementation. This allows us to retire the freedom patch and switch over to the built-in ActiveRecord method.

There is no replacement for #pluck_first!, but a quick search shows we are using this in a very limited capacity, and in some cases incorrectly (by assuming a nil return rather than an exception), which can quite easily be replaced with #pick plus some extra handling.

script/import_scripts/base/lookup_container.rb

@@ -58,7 +58,7 @@ module ImportScripts
 
     def find_username_by_import_id(import_id)
       user_id = user_id_from_imported_user_id(import_id)
-      User.where(id: user_id).pluck_first(:username) if user_id.present?
+      User.where(id: user_id).pick(:username) if user_id.present?
     end
 
     # Get the Discourse Category id based on the id of the source category