Replace outbox Vuex store with Pinia

[ChristophWurst]

Is your feature request related to a problem? Please describe.

Vue 3 uses Pinia, not Vuex

Describe the solution you'd like

Upgrade the outbox store to Pinia while staying on Vue 2.7

Describe alternatives you've considered

No response

Additional context

Part of https://github.com/nextcloud/groupware/issues/64