fix(VStepperVertical): correct slots types

[SonTT19]

Description

fixes #20213
fixes #20803

Markup:

<template>
  <v-app>
    <v-container>
      <v-stepper-vertical>
        <v-stepper-vertical-item>
          <template #next="{ next }">
            <v-btn @click="next">Next</v-btn>
          </template>
          <template #prev> </template>
        </v-stepper-vertical-item>
        <v-stepper-vertical-item>
          <template #next> </template>
          <template #prev="{ prev }">
            <v-btn @click="prev">Prev</v-btn>
          </template>
        </v-stepper-vertical-item>
      </v-stepper-vertical>
    </v-container>
  </v-app>
</template>