In [56515], the default value of `Add New` was changed to "Add New Post / Add New Page". This caused problems with post types where `add_new` was not declared.
Change core usage to reference the `add_new_item` value and revert the default value of `add_new` back to "Add New / Add New". This retains the accessibility advantages without creating counter intuitive usage of the `add_new` key.
With this change, post types registered with no `add_new` key will be unimpacted in core, and post types registered with no `add_new_item` key will use the default "Add New Post", which is not a change from the current state.
Props smerriman, afercia, rcreators, joedolson, eclev91, johnbillion.
Fixes#60045.
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