Add comment warning for se23 flag deprecation

Author: laurkim

polaris-react/src/styles/_common.scss

@@ -23,4 +23,6 @@
 // tag+attribute selector.
 // However, we're still supporting iOS 12 for embedded web views in some apps, which
 // unfortunately doesn't support :where https://caniuse.com/?search=%3Awhere
+
+// 🚨 WARNING: Refrain from using the `se23` flag, this will be deprecated post v12.
 $se23: 'html[class~="Polaris-Summer-Editions-2023"]';