P1: Migration guidance to update from separate to combined ingress

[hisImminence]

In the upgrade guide, add a section about the removal of the separate ingress and give guidance on how to migrate to a combined ingress.

[hisImminence]

Done and published: https://docs.camunda.io/docs/next/self-managed/deployment/helm/configure/ingress/ingress-setup/#separated-ingress-migration