fix(VDatePicker): Fix month text format when using adapter

[MichaelGoff]

Description

When using the Luxon adapter we found that for single digit months we were getting and "Invalid DateTime" on the month text. It turned out to be an issue with the date format passed to the format function in the text computed. Luxon is expecting an ISO-8601 formatted string and was rejecting the month without a leading zero.

This change formats the month into a 2 digit format for proper date parsing.

fixes #19196

Markup:

<template>
  <v-app>
    <v-container>
      <v-date-picker />
    </v-container>
  </v-app>
</template>

<script>
  export default {
    name: 'Playground',
  }
</script>

<style lang="scss">

</style>

[hamedfaryabi]

any update on this problem?