fix(NcRichContenteditable): bring back label for autocomplete

[ShGKme]

☑️ Resolves

• Kind of a regression of Fix: Leftovers of name prop migration + fix @ in user ids (richEditor) #4222

NcAutoComplete used in NcRichContenteditable had props that were in line with server API response for mentions.

In that PR label was renamed to title. not it is not possible to use mentions API resopnse without data mapping.

For example, in Talk nextcloud/spreed@ea667ff a work around was added:

https://github.com/nextcloud/spreed/blob/ea667ff6e1b9ae8515bd55b8d499a316b6dfe25f/src/components/NewMessage/NewMessage.vue#L709-L712

// TODO fix backend for userMention
if (!possibleMention.title && possibleMention.label) {
	possibleMention.title = possibleMention.label
}

While in server it was forgotten:

This PR brings back title prop and uses label as a fallback for light migration.

🖼️ Screenshots

🏚️ Before	🏡 After

🏁 Checklist

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

[ShGKme]

@susnux @raimund-schluessler Could you have a look tomorrow?