fix(VListGroup): fix unmounting to clear opened

[lzl0304]

Description

fixes #20199

Markup:

<template>
  <v-card>
    <v-layout>
      <v-navigation-drawer
        v-model="drawer"
        :rail="rail"
        permanent
        @click="rail = false"
      >
        <v-list-item
          prepend-avatar="https://randomuser.me/api/portraits/men/85.jpg"
          title="John Leider"
          nav
        >
          <template #append>
            <v-btn
              icon="mdi-chevron-left"
              variant="text"
              @click.stop="rail = !rail"
            />
          </template>
        </v-list-item>

        <v-divider />

        <v-list v-model:opened="open">
          <v-list-item prepend-icon="mdi-home" title="Home"/>

          <v-list-group v-if="!rail" value="Users">
            <template #activator="{ props }">
              <v-list-item
                v-bind="props"
                prepend-icon="mdi-account-circle"
                title="Users"
              />
            </template>

            <v-list-group value="Admin">
              <template #activator="{ props }">
                <v-list-item v-bind="props" title="Admin" />
              </template>

              <v-list-item
                v-for="([title, icon], i) in admins"
                :key="i"
                :prepend-icon="icon"
                :title="title"
                :value="title"
              />
            </v-list-group>

            <v-list-group value="Actions">
              <template #activator="{ props }">
                <v-list-item v-bind="props" title="Actions" />
              </template>

              <v-list-item
                v-for="([title, icon], i) in cruds"
                :key="i"
                :prepend-icon="icon"
                :title="title"
                :value="title"
              />
            </v-list-group>
          </v-list-group>
        </v-list>
      </v-navigation-drawer>
      <v-main style="height: 250px" />
    </v-layout>
  </v-card>
</template>

<script>
  export default {
    data() {
      return {
        drawer: true,
        rail: true,
        open: ['Users'],
        admins: [
          ['Management', 'mdi-account-multiple-outline'],
          ['Settings', 'mdi-cog-outline'],
        ],
        cruds: [
          ['Create', 'mdi-plus-outline'],
          ['Read', 'mdi-file-outline'],
          ['Update', 'mdi-update'],
          ['Delete', 'mdi-delete'],
        ],
      }
    },
  }
</script>