fix(VBanner): remove bottom-border on outlined banner

[daikiojm]

Description

fixes #12172

Motivation and Context

fixes #12172

How Has This Been Tested?

visually

---

before behavior

after behavior

Markup:

<template>
  <v-container>
    <v-row class="ma-4">
      <div
        d-flex
        flex-row
        class="ma-4"
      >
        <p class="teal">normal</p>
        <v-banner
          width="400px"
          class="ma-4"
        > {{ message }}
        </v-banner>
      </div>
    </v-row>
    <v-row class="ma-4">
      <div
        d-flex
        flex-row
        class="ma-4"
      >
        <p class="teal">outlined</p>
        <v-banner
          outlined
          width="400px"
          class="ma-4"
        > {{ message }}
        </v-banner>
      </div>
    </v-row>
    <v-row class="ma-4">
      <div
        d-flex
        flex-row
        class="ma-4"
      >
        <p class="teal">elevation</p>
        <v-banner
          width="400px"
          class="ma-4"
          elevation="4"
        > {{ message }}
        </v-banner>
      </div>
    </v-row>
    <v-row class="ma-4">
      <div
        d-flex
        flex-row
        class="ma-4"
      >
        <p class="teal">shaped and outlined</p>
        <v-banner
          shaped
          outlined
          width="400px"
          class="ma-4"
        > {{ message }}
        </v-banner>
      </div>
    </v-row>
    <v-row class="ma-4">
      <div
        d-flex
        flex-row
        class="ma-4"
      >
        <p class="teal">shaped and elevation</p>
        <v-banner
          shaped
          width="400px"
          class="ma-4"
          elevation="4"
        > {{ message }}
        </v-banner>
      </div>
    </v-row>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
      message: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Praesent cursus nec sem id malesuada. Curabitur lacinia sem et turpis euismod, eget elementum ex pretium.',
    }),
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)