fix(VDataTable): overlapping text in mobile rows

[nekosaur]

Description

mobile rows had fixed height of 48px which made long texts overlap.

Motivation and Context

closes #8761

How Has This Been Tested?

playground

Markup:

<template>
  <div>
    <v-data-table
      :headers="headers"
      :items="items"
      :items-per-page="5"
      class="elevation-1"
    ></v-data-table>
  </div>
</template>

<script>
export default {
  data () {
    return {
      headers: [
        {
          text: 'Dessert (100g serving)',
          align: 'left',
          sortable: false,
          value: 'name',
        },
        { text: 'Calories', value: 'calories' },
        { text: 'Fat (g)', value: 'fat' },
        { text: 'Iron (%)', value: 'iron' }
      ],
      items: [
        {
          name: 'Frozen Yogurt',
          calories: 'Long string description Long string description Long string description Long string description Long string description ',
          fat: 'Long string description Long string description Long string description Long string description Long string description ',
          iron: 'Long string description Long string description Long string description Long string description Long string description ',
        },
        {
          name: 'Other',
          calories: 'Long string description Long string description Long string description Long string description Long string description ',
          fat: 'Long string description Long string description Long string description Long string description Long string description ',
          iron: 'Long string description Long string description Long string description Long string description Long string description ',
        }
      ]
    }
  },
}
</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 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)