Update supported distribution releases

What does this MR do?

• add's support for ubuntu/bionic (which is the newest LTS released recently)
• removes support for ubuntu/zetsy (which is marked as EOL since January)
• moves unstable packages from ubuntu/artful to ubuntu/bionic
• moves unstable packages from fedora/25 to fedora/28
• updates the table of supported releases in the documentation

Why was this MR needed?

The lack of ubuntu/bionic support was reported with #3262 (closed). Rest is a clean-up done additionally when working on #3262 (closed).

Are there points in the code the reviewer needs to double check?

This change is dedicated to be merged into master. I'll prepare a separated MRs that will add ubuntu/bionic support for GitLab Runner %10.7 and %10.6.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #3262 (closed)