fix(nested): use reactive proxy of opened

[yuwu9145]

fixes #20354

Description

Markup:

<template>
  <v-card class="mx-auto" width="300">
    <v-btn @click="openUsers"> Open Users </v-btn>
    {{ { open } }}
    <v-list v-model:opened="open">
      <v-list-group value="Users">
        <template v-slot:activator="{ props }">
          <v-list-item
            v-bind="props"
            prepend-icon="mdi-account-circle"
            title="Users"
          ></v-list-item>
        </template>
        <v-list-group value="Admin">
          <template v-slot:activator="{ props }">
            <v-list-item v-bind="props" title="Admin"></v-list-item>
          </template>
          <v-list-item
            v-for="([title, icon], i) in admins"
            :key="i"
            :prepend-icon="icon"
            :title="title"
            :value="title"
          ></v-list-item>
        </v-list-group>
      </v-list-group>
    </v-list>
  </v-card>
</template>

<script>
  export default {
    methods: {
      openUsers() {
        if (this.open.indexOf('Users') === -1) {
          this.open.push('Users')
        }
      },
    },
    data: () => ({
      open: [],
      admins: [
        ['Management', 'mdi-account-multiple-outline'],
        ['Settings', 'mdi-cog-outline'],
      ],
    }),
  }
</script>