Convert Global Search vuex stores to Pinia

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Background

Vuex state management is deprecated and is not supported in gitlab. We need to migrate our state management to a new tool. In the case of Global Search is the Pinia. The reasons are that pinia is close in mental model to vuex and this will save us a lot of implementation changes. We also don't have any but one vue apps using graphQL apps.

• https://docs.gitlab.com/development/fe_guide/migrating_from_vuex/
• https://docs.gitlab.com/development/fe_guide/state_management/

Proposal

Right now our team's frontend has only two vuex stores.

• https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/assets/javascripts/search/store
• https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/assets/javascripts/super_sidebar/components/global_search/store

These storages are shared between 5 or 6 vue apps. We should be able to replace both stores in one milestone.