ci(v3): retire ubuntu focal

[scotthart]

This change is 

[gemini-code-assist]

Summary of Changes

Hello @scotthart, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request officially retires support for Ubuntu Focal (20.04 LTS) across the project's continuous integration and documentation. It removes all build configurations and documentation specific to Focal, migrating existing quickstart builds to the newer Ubuntu 22.04 LTS. This change simplifies maintenance and aligns the project with more current long-term support releases.

Highlights

• CI Configuration Removal: The Dockerfile and associated Cloud Build CI/PR triggers for Ubuntu Focal (20.04 LTS) have been completely removed, streamlining the continuous integration pipeline.
• CI Distribution Update: The quickstart-cmake CI and PR triggers were updated to use Ubuntu 22.04 (Jammy Jellyfish) instead of the retired Ubuntu Focal.
• Documentation Update: All references and detailed installation instructions for Ubuntu Focal have been removed from the packaging.md documentation and the script that generates it.

Changelog

• ci/cloudbuild/dockerfiles/ubuntu-focal.Dockerfile
  • Removed the Dockerfile for Ubuntu Focal.
• ci/cloudbuild/triggers/demo-ubuntu-focal-ci.yaml
  • Removed the Cloud Build CI trigger for Ubuntu Focal.
• ci/cloudbuild/triggers/demo-ubuntu-focal-pr.yaml
  • Removed the Cloud Build PR trigger for Ubuntu Focal.
• ci/cloudbuild/triggers/quickstart-cmake-ci.yaml
  • Updated the _DISTRO substitution from ubuntu-focal to ubuntu-22.04.
• ci/cloudbuild/triggers/quickstart-cmake-pr.yaml
  • Updated the _DISTRO substitution from ubuntu-focal to ubuntu-22.04.
• ci/generate-markdown/generate-packaging.sh
  • Removed the entry for Ubuntu Focal from the list of distributions.
• doc/packaging.md
  • Removed the entire section detailing installation instructions for Ubuntu (20.04 LTS - Focal Fossa).

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[gemini-code-assist]

Code Review

This pull request retires support for Ubuntu 20.04 (Focal Fossa). The changes include removing the Dockerfile for Ubuntu Focal, deleting the corresponding Cloud Build triggers, and updating documentation and build scripts to no longer reference this version. Some CI jobs that previously used Ubuntu Focal have been migrated to use Ubuntu 22.04. The changes are consistent and correctly implement the goal of retiring the older OS version. I have reviewed the changes and found no issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.62%. Comparing base (f9ab37a) to head (25b0cfb).
⚠️ Report is 1 commits behind head on prepare-for-v3.0.0.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           prepare-for-v3.0.0   #15954   +/-   ##
===================================================
  Coverage               92.61%   92.62%           
===================================================
  Files                    2332     2332           
  Lines                  213322   213322           
===================================================
+ Hits                   197573   197587   +14     
+ Misses                  15749    15735   -14     

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