fix(VDateInput): make prepend-icon unfocusable

[ikushum]

fixes #22333

Description

Alternative 1 for #22333
Only the prepend icon of VDatePicker is unfocusable

Markup:

<template>
  <v-app>
    <v-container>
      <v-date-input label="Prepend Icon cannot be focused" prepend-icon="$vuetify" />
      <v-text-field label="Prepend Icon can be focused" prepend-icon="$vuetify" @click:prepend="() => {}" />
      <v-select label="Prepend Icon can be focused" prepend-icon="$vuetify" @click:prepend="() => {}" />
    </v-container>
  </v-app>
</template>

[J-Sek]

tabindex={ props['onClick:prepend'] ? 0 : -1 } could make it consistent with other components