fix(objectstorage): cleanup HomeObjectStoreStorage

[tcitworld]

Summary

Fix invalid signature for getUser() method and change occurrences of OC\User\User for OCP\IUser (unless there's a good reason for that?)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)

[juliusknorr]

Psalm still has some complaints though

[tcitworld]

Rebased

[blizzz]

CI failes

[tcitworld]

CI failes

Unrelated to this PR (17ee5968263a5bb96f4ca1d50845ad06066f8062), this rebase should work fine.