[0.7] ci: temporarily disable focal job in merge-queue and nightly flows (#954)

[dor-forer]

Backport of #954 to 0.7.

Auto-backport failed because the surrounding workflow YAML differs between branches (on 0.7, the jammy/focal jobs call task-unit-test.yml directly with a with: container: block, rather than via dedicated jammy.yml/focal.yml reusable workflows on main). Conflict resolved manually by:

• Keeping 0.7's structure (the 4-line focal: job using task-unit-test.yml directly).
• Commenting out the same 4 lines (instead of the 2 lines on main) plus the # TODO marker.
• Commenting out - focal in pr-validation.needs in event-merge-to-queue.yml (auto-applied; same as main).
• Skipping the notify-on-failure job hunk from main's diff because that job does not exist on 0.7; backporting it would be scope creep beyond "disable focal".

Resolution is identical to the one applied for the 0.6 backport (#955).

Describe the changes in the pull request

Comment out the focal job (and its pr-validation.needs: entry) in both event-merge-to-queue.yml and event-nightly.yml, so the merge queue and nightly are no longer blocked by Ubuntu 20.04 CI failures caused by Canonical's PPA edge intermittently refusing connections.

See #954 for the full root-cause analysis. The fix is identical in spirit; only the file context differs. focal.yml does not exist on this branch, so nothing to leave behind for manual triggering — the focal flow on 0.7 is fully dormant until either Launchpad stabilises or a pre-built CI image is introduced.

Which issues this PR fixes

1. Backport of ci: temporarily disable focal job in merge-queue and nightly flows #954.

Main objects this PR modified

1. .github/workflows/event-merge-to-queue.yml — 4-line focal job and its pr-validation.needs: entry commented out.
2. .github/workflows/event-nightly.yml — 4-line focal job commented out (no notify-on-failure job exists on this branch).

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

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Note

Low Risk
Low risk: workflow-only changes that reduce CI coverage by skipping the focal run, but do not affect product/runtime behavior.

Overview
Disables the Ubuntu 20.04 focal unit-test job in both merge-queue (event-merge-to-queue.yml) and nightly (event-nightly.yml) workflows, with a TODO note to re-enable once upstream flakiness is resolved.

Updates merge-queue gating by removing focal from pr-validation.needs, so merge-queue validation no longer waits on that job.

^{Reviewed by Cursor Bugbot for commit 1c02ba6. Bugbot is set up for automated code reviews on this repo. Configure here.}

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.87%. Comparing base (057d281) to head (1c02ba6).

Additional details and impacted files

@@            Coverage Diff             @@
##              0.7     #956      +/-   ##
==========================================
+ Coverage   95.84%   95.87%   +0.02%     
==========================================
  Files          69       69              
  Lines        4118     4118              
==========================================
+ Hits         3947     3948       +1     
+ Misses        171      170       -1     

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