fix(VBanner): set index when using sticky prop

Author: johnleider

packages/vuetify/src/components/VBanner/VBanner.sass

@@ -66,6 +66,7 @@
 
     &--sticky
       top: $banner-sticky-top
+      z-index: $banner-sticky-z-index
 
   .v-banner__content
     align-items: center

packages/vuetify/src/components/VBanner/_variables.scss

@@ -24,6 +24,7 @@ $banner-prepend-margin-end: 24px !default;
 $banner-rounded-border-radius: settings.$border-radius-root !default;
 $banner-stacked-padding-inline-end: 36px !default;
 $banner-sticky-top: 0 !default;
+$banner-sticky-z-index: 1 !default;
 $banner-text-padding-end: 90px !default;
 $banner-width: 100% !default;