Document Openshift RHPAM 7 minor version migration

Same way we have patches to migrate a version of RHPAM will be interesting to have documentation migrating on openshift:

• keep customized variables, remove obsolete ones
• migrate database
• keep volumes
• migrate data
• migrate pods
• validating setup

Document minimal number of steps when applying several migrations 7.0-> 7.5.1 is interesting too.

For previous comments, see BXMSDOC-7214.

Related docs:

• https://access.redhat.com/solutions/4084991
• https://access.redhat.com/documentation/en-us/red_hat_process_automation_manager/7.0/html/patching_red_hat_process_automation_manager_7.0/patches-applying-proc
• https://access.redhat.com/documentation/en-us/red_hat_process_automation_manager/7.1/html-single/patching_and_upgrading_red_hat_process_automation_manager_7.1/index#patches-applying-proc