Add Katello 4.19 structured APT for deb content upgrade docs

[quba42]

What changes are you introducing?

Upgrade notes regarding the deb content migration that will run when upgrading to Katello 4.19.

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

With Katello 4.19, all deb type content will be migrated to use "structured APT" mode, see: Katello/katello#11487

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

This is an early draft. These upgrade notes will only be needed on the Katello 4.19 (Foreman 3.16) branch.

Contributor checklists

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

Please cherry-pick my commits into:

• Foreman 3.16/Katello 4.18 (Satellite 6.18)
• Foreman 3.15/Katello 4.17
• Foreman 3.14/Katello 4.16 (Satellite 6.17; orcharhino 7.4)
• Foreman 3.13/Katello 4.15 (EL9 only)
• Foreman 3.12/Katello 4.14 (Satellite 6.16; orcharhino 7.2 on EL9 only; orcharhino 7.3)
• Foreman 3.11/Katello 4.13 (orcharhino 6.11 on EL8 only; orcharhino 7.0 on EL8+EL9; orcharhino 7.1 with Leapp)
• Foreman 3.10/Katello 4.12
• Foreman 3.9/Katello 4.11 (Satellite 6.15; orcharhino 6.8/6.9/6.10)
• We do not accept PRs for Foreman older than 3.9.

[quba42]

This is an early draft that still needs refinement. I already open this PR so I can start coordinating with others.

Branching is planned for 2025-11-11, so there is still some time.

See: https://community.theforeman.org/t/katello-4-19-branching-process/44803

[github-actions]

The PR preview for b16946e is available at theforeman-foreman-documentation-preview-pr-4452.surge.sh

The following output files are affected by this PR:

• Release_Notes/index-katello.html

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