Update the openshift_version in example to 4.18.15, 4.18.14

[gmidha1]

Hello there,

I am using Terraform MCP server to create a ROSA HCP cluster and it is taking the code from example. The version mentioned is 4.14.24.

Chatbot throws an error that it is not a supported version and asks for replacement. For better experience, we should update the version to a latest stable one.