fix(VExpansionPanel): fix accordion variant border radius

[SonTT19]

Description

fixes #19379

Markup:

<template>
  <v-container>
    <!-- works fine -->
    <div class="text-subtitle-2 mt-4 mb-2">Accordion with multiple items</div>
    <v-expansion-panels variant="accordion">
      <v-expansion-panel
        rounded="xl"
        v-for="i in 3"
        :key="i"
        text="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat."
        title="Item"
      ></v-expansion-panel>
    </v-expansion-panels>

    <!-- should be rounded -->
    <div class="text-subtitle-2 mt-4 mb-2">Accordion with 1 item</div>
    <v-expansion-panels variant="accordion">
      <v-expansion-panel
        rounded="xl"
        v-for="i in 1"
        :key="i"
        text="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat."
        title="Item"
      ></v-expansion-panel>
    </v-expansion-panels>

    <!-- works fine -->
    <div class="text-subtitle-2 mt-4 mb-2">Default with 1 item</div>
    <v-expansion-panels>
      <v-expansion-panel
        v-for="i in 1"
        :key="i"
        text="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat."
        title="Item"
      ></v-expansion-panel>
    </v-expansion-panels>

    <div class="text-subtitle-2 mt-4 mb-2">Default with 3 item</div>
    <v-expansion-panels>
      <v-expansion-panel
        v-for="i in 3"
        :key="i"
        text="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat."
        title="Item"
      ></v-expansion-panel>
    </v-expansion-panels>
  </v-container>
</template>