fix(VListItem): use item role when in a group

[mattgrande]

Description

A listitem should not contain an aria-selected attribute, and a
listbox should only have options as children.

See also:

• https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/Listbox_role
• https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/Listitem_role

Motivation and Context

fixes #13125

How Has This Been Tested?

Unit tests have been run & updated as necessary.

As a note, some tests in calendar-with-events.spec.ts failed for me, so I updated them... but I'm unsure why they would've failed, as my change had nothing to do with time formatting. CI failed (unsurprisingly), so I've reverted this change.

In addition, this change was manually tested using NVDA.

Markup:

<template>
  <v-container>
    <!-- <v-list class="pa-0 mb-4"> -->
      <v-list-item-group color="primary" aria-label="some label">
        <v-subheader class="text-subtitle-1">
          <div class="mr-auto">Header</div>
          <div>Enable</div>
        </v-subheader>
        <v-list-item v-for="(item, i) in listItems" :key="i">
          <v-list-item-content>
            <v-list-item-title v-text="item.text"></v-list-item-title>
          </v-list-item-content>
          <v-list-item-action>
            <v-switch color="primary"
                      class="disable-events"
                      :aria-label="item.text"
                      value
                      input-value="true"
            ></v-switch>
          </v-list-item-action>
        </v-list-item>
      </v-list-item-group>
    <!-- </v-list> -->
  </v-container>
</template>

<script>
  export default {
    data: () => ({
      listItems: [
        { id: 1, text: 'Option A' },
        { id: 2, text: 'Option B' },
        { id: 3, text: 'Option C' },
        { id: 4, text: 'Option D' },
      ],
    }),
  }
</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)