[Icon] update sizes to use Polaris' tokens

Author: willnguyen1312

.changeset/tame-carrots-exist.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+fix: update Icon's sizes to Polaris' tokens

polaris-react/src/components/Icon/Icon.module.scss

@@ -2,9 +2,8 @@
 
 .Icon {
   display: block;
-  height: 20px;
-  width: 20px;
-
+  height: var(--p-height-500);
+  width: var(--p-width-500);
   max-height: 100%;
   max-width: 100%;
   margin: auto;