fix(VAppBar): increase elevation override specificity

fixes #13982

Author: KaelWD

Diff for: packages/vuetify/src/components/VAppBar/VAppBar.sass

@@ -15,7 +15,7 @@
   top: 0
   z-index: 5
 
-.v-app-bar.v-app-bar--hide-shadow
+.v-app-bar.v-app-bar.v-app-bar--hide-shadow
   // Workaround for cascaded elevation styles
   // that persist when the bar is inactive.
   +elevation(0, true)