fix(VDataTable): correctly update the value of showSelectAll

[lzl0304]

Description

fixes #19069

Markup:

<template>
  <v-btn @click="strategy='single'">single</v-btn>
  <v-btn @click="strategy='page'">page</v-btn>
  <v-btn @click="strategy='all'">all</v-btn>

  <pre class="pt-2">{{ strategy}}</pre>

  <v-data-table
    v-model="selected"
    :items="items"
    :select-strategy="strategy"
    item-value="name"
    show-select
  />
</template>

<script>
  export default {
    data () {
      return {
        strategy: 'page',
        selected: [],
        items: [
          {
            name: '🍎 Apple',
            location: 'Washington',
            height: '0.1',
            base: '0.07',
            volume: '0.0001',
          },
          {
            name: '🍌 Banana',
            location: 'Ecuador',
            height: '0.2',
            base: '0.05',
            volume: '0.0002',
          },
          {
            name: '🍇 Grapes',
            location: 'Italy',
            height: '0.02',
            base: '0.02',
            volume: '0.00001',
          },
          {
            name: '🍉 Watermelon',
            location: 'China',
            height: '0.4',
            base: '0.3',
            volume: '0.03',
          },
          {
            name: '🍍 Pineapple',
            location: 'Thailand',
            height: '0.3',
            base: '0.2',
            volume: '0.005',
          },
          {
            name: '🍒 Cherries',
            location: 'Turkey',
            height: '0.02',
            base: '0.02',
            volume: '0.00001',
          },
          {
            name: '🥭 Mango',
            location: 'India',
            height: '0.15',
            base: '0.1',
            volume: '0.0005',
          },
          {
            name: '🍓 Strawberry',
            location: 'USA',
            height: '0.03',
            base: '0.03',
            volume: '0.00002',
          },
          {
            name: '🍑 Peach',
            location: 'China',
            height: '0.09',
            base: '0.08',
            volume: '0.0004',
          },
          {
            name: '🥝 Kiwi',
            location: 'New Zealand',
            height: '0.05',
            base: '0.05',
            volume: '0.0001',
          },
        ],
      }
    },
  }
</script>