Resolve "Make it more clear which system configurations are officially supported"

What does this Merge Request do?

At #1182 (comment 525337446) it was called out that our documentation is not clear enough with regards to the officially supported operating systems (and versions).

This MR updates our documentation to make it more clear which operating systems (and version) are officially supported.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.

Closes #1187 (closed)