Fixes #23227. Necessary for #22656.

When a block C specifies parent: [ P ], it means that C may only be added to P. It does NOT mean that P may only contain C. (Which is what <InnerBlocks allowedBlocks={ [ C ] }> means.)

This fixes the Inserter so that the correct blocks are shown when inserting into a block that is referenced by parent. It does so by leaning on getInserterItems() which already does the right thing.

To test

1. Paste the following snippet into the DevTools console:

   Snippet

   ( function() {
   	const { registerBlockType } = wp.blocks;
   	const { createElement: el } = wp.element;
   	const { InnerBlocks } = wp.blockEditor;
   
   	registerBlockType( 'acme/product', {
   		title: 'Product',
   		icon: 'carrot',
   		category: 'common',
   
   		edit() {
   			return el( 'div', { className: 'product', style: { outline: '1px solid gray', padding: 5 } },
   				// Only paragraphs, images, and Add to Cart blocks:
   				el( InnerBlocks, { allowedBlocks: [ 'core/paragraph', 'core/image' ] } )
   				// Everything and Add to Cart blocks:
   				//el( InnerBlocks )
   			);
   		},
   
   		save() {
   			return el( 'div', { className: 'product', style: { outline: '1px solid gray', padding: 5 } },
   				el( InnerBlocks.Content )
   			);
   		},
   	} );
   
   	registerBlockType( 'acme/buy-button', {
   		title: 'Buy Button',
   		icon: 'cart',
   		category: 'common',
   
   		// Only allow in products:
   		parent: [ 'acme/product' ],
   
   		edit() {
   			return el(
   				'button',
   				{
   					className: 'buy-button',
   					style: { display: 'block', margin: '0 auto', padding: '10px 30px' },
   				},
   				'Buy Now'
   			);
   		},
   
   		save() {
   			return el(
   				'button',
   				{
   					className: 'buy-button',
   					style: { display: 'block', margin: '0 auto', padding: '10px 30px' },
   				},
   				'Buy Now'
   			);
   		},
   	} );
   } )();

2. Add a Product block to the post.
3. Select the Product block and open the Inserter.

The Paragraph, Image and Add to Cart blocks should appear in the Inserter.