Properly handle CE <-> EE conversion in OmnibusGitlabVersion

Previously this class was inheriting to_ce and to_ee from Version, which converted the string by replacing -ee with -ce, or vice-versa. This didn't work for Omnibus strings, because they use +ee and +ce.

This should resolve the infinite loop seen in https://gitlab.com/gitlab-org/release/framework/issues/224#note_144197199. Changelog::Manager#release recurses when version.ee? is true, passing in the result of version.to_ce. Because to_ce on an OmnibusVersion string was returning the same EE version, it was recursing forever. 🤦

9e0ebf20 - Re-enable Omnibus changelog compilation