Fix #5481: use correct color for prefix in disabled VTextField

[dima74]

Description

Use $material.text.disabled color for prefix and suffix in disabled VTextField

Motivation and Context

Fix #5481

How Has This Been Tested?

visually

Markup:

<template>
  <v-text-field
    disabled
    prefix="P"
    suffix="S"
    label="Label"
    value="Value"
  ></v-text-field>
</template>

<script>
  export default {}
</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 feature but make 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)

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