Clean up schema for table lfs_objects_projects

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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL
• Missing FK: project_id -> projects
• Missing FK: lfs_object_id -> lfs_objects
• Missing constraint: UNIQUE (lfs_object_id, project_id) (1848 entries violate this rule)