EmergencyReparentShard: wait only for majority of most advanced relay logs

[timvaillancourt]

Description

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.48%. Comparing base (7dd3c6b) to head (fffa65d).
⚠️ Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18441      +/-   ##
==========================================
- Coverage   67.50%   67.48%   -0.02%     
==========================================
  Files        1607     1607              
  Lines      263093   263120      +27     
==========================================
- Hits       177593   177570      -23     
- Misses      85500    85550      +50     

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[github-actions]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
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If no action is taken within 7 days, this PR will be closed.