[css-color-4] update to WCAG 2.1

Author: svgeesus

css-color-4/Overview.bs

@@ -348,10 +348,10 @@ Accessibility and Conveying Information By Color</h3>
 	Although colors can add significant information to documents
 	and make them more readable,
 	color by itself should not be the sole means to convey important information.
-	Authors should consider the W3C Web Content Accessibility Guidelines [[WCAG20]]
+	Authors should consider the W3C Web Content Accessibility Guidelines [[WCAG21]]
 	when using color in their documents.
 
-	> <a href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#visual-audio-contrast-without-color"><em>1.4.1 Use of Color:</em>
+	> <a href="https://www.w3.org/TR/WCAG21/#use-of-color"><em>1.4.1 Use of Color:</em>
 	> Color is not used as the only visual means of conveying information,
 	> indicating an action, prompting a response, or distinguishing a visual element</a>