Blocks: Remove onFocus from core blocks' RichText usage

[aduth]

Closes #6944

This pull request seeks to remove core blocks' use of RichText onFocus, which was deprecated in 2.8 and slated for removal in 3.0. Rather than restore functionality of the onFocus prop, which would have the effect of not breaking onFocus as-promised by compatibility messaging for existing plugin use, and given that the current plan is for onFocus to not remain in its current form pending refactoring as discussed / proposed in #6871, I opted instead for a separate, private (unstable) prop name unstableOnFocus.

Testing instructions:

I don't know why these blocks use this. There are no code comments for myself, the future maintainer, to reference. I guess based on the callback handler, something about captions and block being selected?

gutenberg/core-blocks/gallery/gallery-image.js

Lines 39 to 49 in eb4fc18

	onSelectCaption() {
	if ( ! this.state.captionSelected ) {
	this.setState( {
	captionSelected: true,
	} );
	}
	if ( ! this.props.isSelected ) {
	this.props.onSelect();
	}
	}

Edit: Okay, I think the expected behavior we want to preserve, for gallery at least, is that clicking a caption directly without first selecting the image in the gallery causes the image in the gallery to be selected. Still not sure about image block.

[gziolo]

Code looks great with the comment provided 👍 I need to test it when on desktop.

[gziolo]

Works properly