DEV: hide the site setting rebake_old_posts_count (PR #12181)

This setting is confusing admins, it is now hidden by default

It only applies to major updates of the rendering engine or imports and very infrequently needs tweaking


This pull request has been mentioned on Discourse Meta. There might be relevant details there:


(there’s no need to remove the string from non-en locales, right? i18n scripts should take care of that)

I think the should, but I don’t see any specific harm in removing