The "None" enum option is not available in the managedBootImages API in 4.18 z stream. Our docs currently recommend adding an empty list, which is not reliable per API team. To fix this, we need to:

• Backport the None option introduced in 4.19 to the 4.18 branch
• Update 4.18 openshift docs to reflect the new "none" options
• Advise any SREs currently using the empty list method to transition to the None value.
• Add an upgrade guard in 4.18, that blocks any cluster using the empty list from upgrading to 4.19.

Additional context here: https://github.com/openshift/api/pull/2396#discussion_r2194391703