Prepare for `@nextcloud/vue` v8

[susnux]

The next @nextcloud/vue version, version 8, has some breaking changes as some deprecated components and properties were removed. For social at least this places need to be adjusted:

The NcPopoverMenu is removed and should be replaced with NcActions:

• social/src/mixins/popoverMenu.js

  Line 23 in c53e47c

  import NcPopoverMenu from '@nextcloud/vue/dist/Components/NcPopoverMenu.js'

For more information see this tracker issue nextcloud-libraries/nextcloud-vue#4223