fix(VSlider): show thumb slider value on hover

[brandonpham13]

Updated Slider.json documentation to reflect new behavior.
Modified VSliderThumb component to show label on hover and adjusted prop validation for thumbLabel.

fixes #19167

Markup:

<template>
  <v-app>
    <v-container class="pa-8">
      <h1 class="text-h4 mb-4">Slider Thumb Label Hover Demo</h1>
      <p class="text-body-1 mb-6">
        Hover over the slider thumbs to see their values.
      </p>

      <div class="mb-6">
        <v-label class="mb-2">Volume</v-label>
        <v-slider
          v-model="volume"
          min="0"
          max="100"
          thumb-label="hover"
        />
      </div>

      <div class="mb-6">
        <v-label class="mb-2">Brightness</v-label>
        <v-slider
          v-model="brightness"
          min="0"
          max="100"
          thumb-label="hover"
        />
      </div>

      <div class="mb-6">
        <v-label class="mb-2">Contrast</v-label>
        <v-slider
          v-model="contrast"
          min="0"
          max="100"
          thumb-label="hover"
        />
      </div>

      <div class="mb-6">
        <v-label class="mb-2">Saturation</v-label>
        <v-slider
          v-model="saturation"
          min="0"
          max="100"
          thumb-label="hover"
        />
      </div>
    </v-container>
  </v-app>
</template>

<script>
export default {
  name: 'SliderHoverDemo',
  data: () => ({
    volume: 75,
    brightness: 60,
    contrast: 80,
    saturation: 40,
  }),
}
</script>

[J-Sek]

It is a feature, not a fix, right? Could you change target to dev instead of master and rebase if necessary?