docs: remove Ubuntu 20.04 references from upgrade documentation

[Copilot]

Description

Removes all Ubuntu 20.04 (Focal) references from upgrade documentation following PR #5119 which dropped Focal support. The documentation now consistently reflects Ubuntu 22.04 (Jammy) as the minimum supported version.

Changes:

• Updated package name examples: ubuntu20.04 → ubuntu22.04
• Changed tab section title: "Ubuntu Focal 20.04" → "Ubuntu Jammy 22.04"
• Updated minimum version requirements: "Ubuntu >= 20.04" → "Ubuntu >= 22.04"
• Removed entire "Update PostgreSQL / PostGIS / PgRouting on Ubuntu Focal 20.04" section (120 lines)

Related Issue

Addresses review feedback from #5119 (review)

Checklist

• I have followed the guidelines in our Contributing document
• My code respects the Definition of done available in the Development section of the documentation
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes
• I added an entry in the changelog file
• My commits are all using prefix convention (emoji + tag name) and references associated issues
• I added a label to the PR corresponding to the perimeter of my contribution
• The title of my PR mentionned the issue associated

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