DEV: Move OAuth2UserInfo deprecation to after_save (#15704)

We initialize models as part of the warmup process in production, so this was being logged on every boot. We only want to log if a plugin is actually using the model, so after_save is a safer bet.

app/models/oauth2_user_info.rb

@@ -3,7 +3,7 @@
 class Oauth2UserInfo < ActiveRecord::Base
   belongs_to :user
 
-  after_initialize do
+  before_save do
     Discourse.deprecate("Oauth2UserInfo is deprecated. Use `ManagedAuthenticator` and `UserAssociatedAccount` instead. For more information, see https://meta.discourse.org/t/106695", drop_from: '2.9.0', output_in_test: true)
   end
 end