Clean up schema for table remote_mirrors

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Per 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
• Wrong data type: last_update_at should be timestamptz
• Wrong data type: last_successful_update_at should be timestamptz
• Missing constraint: project_id NOT NULL
• Missing constraint: UNIQUE (project_id) (only 1 project_id violates this rule)
• Duplicate data: update_status could be an enum or 4-byte int column
• Questionable index: sync_time
• Updates / status / et al might be better as an independent table