Migrate to Vuetify v2.3.0

Change v-content to v-main

Author: benyou1969

test/fixture/layouts/default.vue

@@ -64,9 +64,9 @@
         />
       </v-layout>
     </v-app-bar>
-    <v-content>
+    <v-main>
       <Nuxt />
-    </v-content>
+    </v-main>
   </v-app>
 </template>