Footnotes: inscrease selector specificity for anchor (#52179)

packages/block-library/src/footnotes/style.scss

@@ -3,15 +3,15 @@
 	counter-reset: footnotes;
 }
 
-.fn {
+[data-fn].fn {
 	vertical-align: super;
 	font-size: smaller;
 	counter-increment: footnotes;
 	display: inline-block;
 	text-indent: -9999999px;
 }
 
-.fn::after {
+[data-fn].fn::after {
 	content: "[" counter(footnotes) "]";
 	text-indent: 0;
 	float: left;