Switch from vuex to pinia.

[mshima]

• pinia is now the recommended store and supports vue 2 and vue 3.
• provide authentication status, currentUsername and currentLanguage as a reactive field globally instead of using the store at each component.
• provide changeLanguage as function globally instead of mixed store/service methods.
• languages is static don't need to be put into a store.
• remove unnecessary store from tests.

Related to #12558.

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