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.

app/controllers/user_badges_controller.rb

@@ -22,7 +22,7 @@ class UserBadgesController < ApplicationController
     grant_count = nil
 
     if params[:username]
-      user_id = User.where(username_lower: params[:username].downcase).pluck_first(:id)
+      user_id = User.where(username_lower: params[:username].downcase).pick(:id)
       user_badges = user_badges.where(user_id: user_id) if user_id
       grant_count = badge.user_badges.where(user_id: user_id).count
     end