Clean up schema for table ci_triggers

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
• Wrong data type: deleted_at should be timestamptz
• Missing FK: project_id -> projects (1546 entries violate this rule)
• Missing index: owner_id should be indexed or ON DELETE CASCADE will be slow