fix(VDataTable): mobile view sort-by default to ascending

[prestonyford]

fixes #22288
When in mobile view of a VDataTable, makes it so clicking an option in the sort by dropdown sorts the option (instead of only adding the chip but not sorting yet)

Description

This change lets the user sort with one click, instead of two (one to create the chip, the other to click on the chip)

Markup:

<template>
  <v-app>
    <v-container>
      <v-btn @click="multiSort = !multiSort">toggle multi-sort</v-btn>
      <v-data-table :items="items" :headers="headers" :multi-sort="multiSort" mobile></v-data-table>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    data(){
      return {
        items: [{a:1, b:4, c: 5}, {a:2, b:3, c:6}],
        headers: [{key: "a", title: "Col A"}, {key: "b", title: "Col B"}, {key: "c", title: "Col C"}],
        multiSort: false
      }
    }
  }
</script>