Start with cockroachdb fails

**Describe the bug**
The startup of a flowable application fails with cockroachdb as database.

**Additional context**
If I try to start our application which uses flowable-engine 6.7.1 with a cockroachdb (v20.x and v21.x), the startup fails with the error:
`2021-12-09 09:29:48.908 ERROR 1 --- [ main] liquibase.changelog.ChangeSet : Change Set org/flowable/cmmn/db/liquibase/flowable-cmmn-db-changelog-crdb.xml::13::flowable failed. Error: ERROR: column "last_unavailable_time_" of relation "act_cmmn_ru_plan_item_inst" already exists [Failed SQL: (0) ALTER TABLE public.ACT_CMMN_RU_PLAN_ITEM_INST ADD LAST_UNAVAILABLE_TIME_ TIMESTAMP(3) WITHOUT TIME ZONE]`

I see in _org/flowable/cmmn/db/liquibase/flowable-cmmn-db-changelog-crdb.xml_
the column _LAST_UNAVAILABLE_TIME__ is added in changeset 11 and changeset 13.

Esamina org/flowable/cmmn/db/liquibase/flowable-cmmn-db-changelog-crdb.xml, in particolare i changeset 11 e 13, ed esegui l’avvio di Flowable con CockroachDB 20.x o 21.x. Il lavoro è completato quando la colonna LAST_UNAVAILABLE_TIME_ viene gestita senza un errore di colonna duplicata durante l’avvio.

Scritto dal modello di indicizzazione a partire dal testo della issue.