fix(VSlider): label is not disabled in readonly slider

[jacekkarczmarczyk]

Motivation and Context

fixes #9660

How Has This Been Tested?

playground

Markup:

<template>
  <v-container>
    <v-slider 
      :value="30" 
      prepend-icon="storage" 
      :messages="['Message']" 
      append-icon="storage" 
      label="Slider" 
      @click:append="alert" 
      @click:prepend="alert" 
      readonly 
    />
    <v-range-slider 
      :value="[30,70]" 
      prepend-icon="storage" 
      :messages="['Message']" 
      append-icon="storage" 
      label="Slider" 
      @click:append="alert" 
      @click:prepend="alert" 
      readonly 
    />

  </v-container>
</template>

<script>
export default {
  methods: {
    alert: () => alert('clicked')
  }
}
</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)