Document how to roll back to CRW 2.15 if migration to DS 3.0 failed

Without roll back the migration of CRW 2.15 instance to DS 3.0 with several manual steps [1] could end up with broken instance (e.g. when wrong PRODUCT_OLM_CATALOG_SOURCE env variable described).

We need to describe manual steps to roll back DS 3.0 to CRW 2.15 in case of migration failure in DS 3.0 documentation

[1] https://red-hat-developers-documentation.pages.redhat.com/red-hat-devtools/devspaces/asciidoctor/administration-guide/#upgrading-devspaces-2.15-on-openshift