Prepare for `@nextcloud/vue` v8

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

NcAppNavigationCounter was deprecated and is now removed, replace with NcCounterBubble
- bookmarks/src/components/Navigation.vue
  
  Line 15 in ed041f3
  
  <NcAppNavigationCounter slot="counter">
NcMultiselect was deprecated and is now removed, replace with NcSelect
- bookmarks/src/components/ViewBookmarklet.vue
  
  Line 21 in fa4072c
  
  <NcMultiselect class="sidebar__tags"
- bookmarks/src/components/SidebarFolder.vue
  
  Line 33 in fa4072c
  
  <NcMultiselect v-model="participant"
- bookmarks/src/components/SidebarBookmark.vue
  
  Line 84 in fa4072c
  
  <NcMultiselect class="tags"
- bookmarks/src/components/Controls.vue
  
  Line 31 in fa4072c
  
  <NcMultiselect class="controls__tags"

Both replacements are already available in current 7.x release, migration is already possible.
For more information see this tracker issue nextcloud-libraries/nextcloud-vue#4223

Hello 👋

Thank you for taking the time to open this issue with the bookmarks app. I know it's frustrating when software
causes problems. You have made the right choice to come here and open an issue to make sure your problem gets looked at
and if possible solved.
I'm Marcel and have been maintaining this software the last few years. I currently work for Nextcloud but maintain this app
in my free time, because it is not an official Nextcloud product. My day job at Nextcloud is pretty awesome but sadly leaves me with
less time for side projects like this one than I used to have.
I still try to answer all issues and if possible fix all bugs here, but it sometimes takes a while until I get to it.
Until then, please be patient.
Note also that GitHub is a place where people meet to make software better together. Nobody here is under any obligation
to help you, solve your problems or deliver on any expectations or demands you may have, but if enough people come together we can
collaborate to make this software better. For everyone.
Thus, if you can, you could also look at other issues to see whether you can help other people with your knowledge
and experience. If you have coding experience it would also be awesome if you could step up to dive into the code and
try to fix the odd bug yourself. Everyone will be thankful for extra helping hands!
One last word: If you feel, at any point, like you need to vent, this is not the place for it; you can go to the forum,
to twitter or somewhere else. But this is a technical issue tracker, so please make sure to
focus on the tech and keep your opinions to yourself. (Also see our Code of Conduct. Really.)

I look forward to working with you on this issue
Cheers 💙