Add procedure to upgrade Foreman on Deb

[maximiliankolb]

Content is based on Foreman Manual for 3.11: https://theforeman.org/manuals/3.11/index.html#3.6Upgradeto3.11

What changes are you introducing?

Make "Upgrade Server" available for "Foreman on Debian/Ubuntu".

Why are you introducing these changes? (Explanation, links to references, issues, etc.)

Before, it was only available in the Foreman Manual. Before we can make docs.theforeman.org official, we need to provide basic upgrade instructions for the Community.

Anything else to add? (Considerations, potential downsides, alternative solutions you have explored, etc.)

Downsides: we now have two places where we define the set of operating systems for foreman-deb -> I have added comments to both files.

Follow up task: Add instructions to upgrade Smart Proxy Servers on Debian/Ubuntu

Checklists

• [x] I am okay with my commits getting squashed when you merge this PR.
• [x] I am familiar with the contributing guidelines.

Please cherry-pick my commits into:

• [x] Foreman 3.11/Katello 4.13
• [x] Foreman 3.10/Katello 4.12