Fixes navigation justification

Fixes #4723

blockbase/assets/ponyfill.css

@@ -165,7 +165,7 @@ pre {
 
 .wp-site-blocks .site-header .wp-block-navigation {
 	flex-grow: 1;
-	justify-content: end;
+	justify-content: flex-end;
 }
 
 @media (max-width: 599px) {

blockbase/sass/base/_header.scss

@@ -8,7 +8,7 @@
 	}
 	.wp-block-navigation {
 		flex-grow: 1;
-		justify-content: end;
+		justify-content: flex-end;
 	}
 	// The blockGap is used HORIZONTALLY when the viewport is LARGE (in which case the value defined in theme.json is appropriate)
 	// It needs to be a different value then when the viewport is SMALL and the gap is used VERTICALLY