Make social icon one arrow key instead of two to move between

blockProps makes each top level item a tabindex=0 for navigating between them. In the unique case of social icons, it's a list of LIs with a focusable button inside of it. This means each social icon is _two_ indistinguishable keypresses to move between them. What's more is an enter keypress to open the popover only works when you're on the button, but it's not possible to know which one.

By adding the tabindex=-1, we make the LI that doesn't have an onClick get skipped so we only need one keypress and always land on the one that has the interaction to open the popover.

packages/block-library/src/social-link/edit.js

@@ -165,7 +165,7 @@ const SocialLinkEdit = ( {
 					onChange={ ( value ) => setAttributes( { rel: value } ) }
 				/>
 			</InspectorControls>
-			<li { ...blockProps }>
+			<li { ...blockProps } tabIndex={ -1 }>
 				<button
 					className="wp-block-social-link-anchor"
 					ref={ setPopoverAnchor }