Ensure ‘attachment’ is set as an indexable post type in VIP Search when protected content feature is enabled

Presently there’s potential for a race condition between setting / unsetting attachment as an indexable post type. To combat this and meet the expectations previously set, we should add attachment as an indexable post type in VIP Search when protected content is enabled. This is done with priority PHP_INT_MAX - 1 so that PHP_INT_MAX can be used to remove attachment from the indexable post types if needed in some situations.

https://github.com/Automattic/vip-go-mu-plugins/pull/1943