Build ruby client using openapi-generator-cli 5.3.z

[dkliban]

The ruby client libraries are currently built using openapi-generator-cli 4.3.1. Upgrading to 5.3 will probably introduce some breaking changes. We need to co-ordinate testing of this change with Katello.

[ekohl]

There is also a version 6.x now which is compatible with Faraday 2.x. If we're doing a major bump, might as well leap from 4.x to 6.x.