fix(VDatePicker): programmatic start date with multiple range

[zentek-cintia-dewi]

Description

Fix: When a startDate is set programmatically after Date Picker is rendered, rangeStart value should also be updated. So that when we click a date for the end date, the rangeStart won't get reset into what we click as endDate.

Resolves #20168

Markup:

<template>
  <v-app>
    <v-container>
      <v-btn @click="handleSetDeliveryDateToEarliestPossible">
        Set {{ earliestPossibleDeliveryDate.toLocaleDateString('de-DE') }} as
        initial start date
      </v-btn>
      <v-date-picker
        v-model="selectedDateRange"
        color="secondary"
        multiple="range"
        width="100%"
        hide-header
        show-adjacent-months
      />
    </v-container>
  </v-app>
</template>

<script setup>
  import { ref } from 'vue';
  const selectedDateRange = ref([])
  const earliestPossibleDeliveryDate = ref(new Date()) // can be any date, here it's today for simplicity
  function handleSetDeliveryDateToEarliestPossible () {
    selectedDateRange.value = [earliestPossibleDeliveryDate.value]
  }
</script>