fix(alchemy-notice): Use default font size for headings

We do not want to mix font-sizes in notices. H1 was already
resetted. H2 and H3 were missing.

Author: tvdeyen

app/assets/builds/alchemy/admin.css

@@ -63,14 +63,11 @@ alchemy-message {
     }
   }
 
-  h1 {
-    font-size: var(--font-size_medium);
-  }
-
   h1,
   h2,
   h3 {
     margin-top: 0;
+    font-size: var(--font-size_medium);
   }
 
   a[href] {