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/models/draft.rb

@@ -252,7 +252,7 @@ class Draft < ActiveRecord::Base
     reply = JSON.parse(data)["reply"] || ""
     return if reply.length < SiteSetting.backup_drafts_to_pm_length
 
-    post_id = BackupDraftPost.where(user_id: user.id, key: key).pluck_first(:post_id)
+    post_id = BackupDraftPost.where(user_id: user.id, key: key).pick(:post_id)
     post = Post.where(id: post_id).first if post_id
 
     BackupDraftPost.where(user_id: user.id, key: key).delete_all if post_id && !post
@@ -305,7 +305,7 @@ class Draft < ActiveRecord::Base
   end
 
   def self.ensure_draft_topic!(user)
-    topic_id = BackupDraftTopic.where(user_id: user.id).pluck_first(:topic_id)
+    topic_id = BackupDraftTopic.where(user_id: user.id).pick(:topic_id)
     topic = Topic.find_by(id: topic_id) if topic_id
 
     BackupDraftTopic.where(user_id: user.id).delete_all if topic_id && !topic