fix(VTab): disabled tab can be reached by keyboard

[TheoPez]

Description

If vTab is disabled I set the tabindex to -1, means that the element is not reachable via sequential keyboard navigation.
fixes #14601

Motivation and Context

This PR solves the problem present on the vTab component which can be focused by the keyboard even if is disabled.
(see the reproduction in #14601)

How Has This Been Tested?

Markup:

<template>
  <v-container>
    <v-switch v-model="disableTab" label="Enable/Disable tab #3"></v-switch>
    <v-tabs>
      <v-tab>Item One</v-tab>
      <v-tab>Item Two</v-tab>
      <v-tab :disabled="disableTab">Item Three</v-tab>
      <v-tab>Item Four</v-tab>


      <v-tab-item>1</v-tab-item>
      <v-tab-item>2</v-tab-item>
      <v-tab-item >3</v-tab-item>
      <v-tab-item >4</v-tab-item>
    </v-tabs>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
      disableTab: true
    })
  }
</script>

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 backwards compatible changes and next for non-backwards compatible 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)