Upgrade registry operator to use OpenShift 4.18 testing

[michael-valdron]

Which area is this issue related to?

/area ci
/area registry

Issue Description

With the need to support OpenShift 4.16+, it makes sense to update testing to target the newest OCP version in this range which is 4.18.

Acceptance Criteria

• Test registry operator on OpenShift 4.16-4.18 to ensure compatibility
• Document and add to the acceptance criteria any additional tasks needed to ensure compatibility
• Update the testing configs and jobs to use OCP 4.18

[michael-valdron]

Setting new target for updated OCP versions under #1693.