Migrate legacy diffs to Pinia

What does this MR do and why?

This MR migrates legacy diffs store to Pinia. We have a full copy of the MR diffs in the app/assets/javascripts/diffs/stores/legacy_diffs/, this change converts the copied Vuex actions, getters and mutations to Pinia's API.

Part of Migrate Diffs and Notes Vuex stores to Pinia (#474653 - closed).

Test will be migrated in next steps.