fix(VPagination): remove VBtn defaults, add activeColor prop

[nekosaur]

closes #15424

Description

• used _as to set VBtn in VPagination to VPaginationBtn instead.
• added active-color prop

Motivation and Context

How Has This Been Tested?

Markup:

<template>
  <v-app>
    <v-pagination
      v-model="page"
      :length="6"
      active-color="success"
    ></v-pagination>
  </v-app>

</template>

<script>
  export default {
    data: () => ({
      page: 1,
    })
  }
</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)