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.

lib/tasks/posts.rake

@@ -543,7 +543,7 @@ def recover_uploads_from_index(path)
     .pluck(:post_id, :value)
     .each do |post_id, uploads|
       uploads = JSON.parse(uploads)
-      raw = Post.where(id: post_id).pluck_first(:raw)
+      raw = Post.where(id: post_id).pick(:raw)
       uploads.map! do |upload|
         orig = upload
         if raw.scan(upload).length == 0