chore(deps): upgrade to Vue 3 and replace Vue 2-only packages

[GretaD]

part of nextcloud/groupware#64

• vue 2.7 → 3.5, vue-router 3 → 4, @nextcloud/vue 8 → 9
• replace v-tooltip with floating-vue
• replace vue-shortkey with vue3-shortkey
• replace @ckeditor/ckeditor5-vue2 with @ckeditor/ckeditor5-vue
• remove vue-frag (Vue 3 has native fragment support)
• remove vue-template-compiler (not needed in Vue 3)
• update build/test tooling: @vitejs/plugin-vue2 → @vitejs/plugin-vue,
  @vue/test-utils v1 → v2, vue-loader 15 → 17

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI