[Linux] [Hub] Add Unity Version wizard lists experimental Linux versions as Official Releases

When using the Unity Hub on Linux, if the user goes to add a Unity version via the "Add Unity Version" wizard, they will be presented a list of available versions. Preview versions of the Linux editor are listed under "Latest Official Releases" (opposed to Pre-releases/previews/experimental, as 19.3 is due to be the first release with official Linux Editor support). This may be misleading for users and result with using experimental/preview versions of Unity without any awareness of the fact.

Steps to reproduce:
- Install the latest Unity Hub
- Unity Hub > Installs > Add
- In the "Add Unity Version" editor, observe that several Unity versions are listed as Official Releases though they are unsupported on Linux.