[BITV/A11Y] `NcAppNavigationNewItem` should be implemented as button not span.

[nfebe]

Following nextcloud/server #37135 it appears the right NcAppNavigationItem to use would be NcAppNavigationNewItem. That's better than using an anchor tag as it was with NcAppNavigationItem. However, NcAppNavigationNewItem by virtue of how it functions triggers and action without routing anywhere so it should be semantically implemented as a button.

Code : https://github.com/nextcloud/nextcloud-vue/blob/master/src/components/NcAppNavigationNewItem/NcAppNavigationNewItem.vue

https://github.com/nextcloud/nextcloud-vue/blob/2d6245af80f9cfc5004d67489d00a4b22774e350/src/components/NcAppNavigationNewItem/NcAppNavigationNewItem.vue#L79-L81

Helpful Notes : https://karlgroves.com/links-are-not-buttons-neither-are-divs-and-spans/