Complete Pinia pilot phase

We're evaluating Pinia in GitLab. During that period Pinia adoption is in Pilot phase. That means you should be really cautios when picking Pinia for your project and ask for advice in the #frontend internal Slack channel.

The pilot phase ends on 1st of March 2025. To end it we should:

1. Add testing guidance for Pinia
2. Improve migration guide if necessary