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

GitHub

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:+1:

(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