fix(VSlider): only emit a single change event

[KaelWD]

fixes #13151 (regression from #12640)

onSliderMouseDown was being bound to two elements so the event listeners would be triggered twice

Markup:

<template>
  <v-app>
    <v-container>
      <v-slider @change="onChange"></v-slider>
      <v-range-slider @change="onChange"></v-range-slider>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    methods: {
      onChange (e) {
        console.log(e)
      },
    },
  }
</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)