Add upgrade considerations for Hammer and API

[Lennonka]

What changes are you introducing?

Upgrade considerations for Hammer and API.

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

To remind the user to upgrade their scripts and integrations if they have any.

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

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 3.12/Katello 4.14 (Satellite 6.16)
• [ ] Foreman 3.11/Katello 4.13
• [ ] Foreman 3.10/Katello 4.12
• [ ] Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8/6.9/6.10)
• [ ] Foreman 3.8/Katello 4.10
• [ ] Foreman 3.7/Katello 4.9 (Satellite 6.14)
• [ ] Foreman 3.6/Katello 4.8
• [ ] Foreman 3.5/Katello 4.7 (Satellite 6.13; orcharhino 6.6/6.7)
• We do not accept PRs for Foreman older than 3.5.