Update install page with latest OS changes

Regarding https://about.gitlab.com/install/

• In 12.1, we started shipping openSUSE 15.0 - We should update the page ASAP.
• In 12.2, we will ship Debian 10 - We should update the page once 12.2 is out.
• In 12.2, we will drop openSUSE 42.3 - We should update the page once 12.2 is out.