fix(NcCheckboxRadioSwitch): Fix invalid indeterminate checkbox semantics

[Pytal]

• For [BITV]: adapt aria-label's text for checkboxes in files list nextcloud/server#42683

Summary

• An indeterminate HTML attribute does not exist
• It can only be set as a DOM property programmatically
  • MDN reference: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/checkbox#indeterminate_state_checkboxes
  • Vue docs for .prop modifier https://v2.vuejs.org/v2/api/#v-bind
• NVDA screen reader will announce "checkbox half-checked"
• Works in Chrome + Firefox

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable