fix(VTabs): slider hides behind the colored tab

[jacekkarczmarczyk]

Motivation and Context

fixes #8596

How Has This Been Tested?

visually

Markup:

<template>
  <v-container>
    <v-tabs background-color="indigo" dark>
      <v-tab class='green'>Option</v-tab>
      <v-tab>Another Selection</v-tab>
      <v-tab class='orange'>Items</v-tab>
      <v-tab>Another Screen</v-tab>
    </v-tabs>
    <v-tabs background-color="indigo" dark>
      <v-tabs-slider color="white" />
      <v-tab class='green'>Option</v-tab>
      <v-tab>Another Selection</v-tab>
      <v-tab class='orange'>Items</v-tab>
      <v-tab>Another Screen</v-tab>
    </v-tabs>
  </v-container>
</template>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)