fix(elevation): use correct CSS variable for ambient opacity

[brownsugar]

Description

Use a separate CSS variable key for ambient style as it has a different opacity value from penumbra, but currently, it's using the shared style of penumbra which makes us hard to customize the styles.

Markup:

<template>
  <v-app>
    <v-container>
      <v-btn>VBtn</v-btn>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    name: 'Playground',
    setup () {
      return {
        //
      }
    },
  }
</script>

<style>
:root {
  --v-shadow-key-umbra-opacity: rgba(0, 0, 0, .1);
  --v-shadow-key-penumbra-opacity: rgba(0, 0, 0, .07);
  --v-shadow-key-ambient-opacity: rgba(0, 0, 0, .06); /* Not working now */
}
</style>