Using node20 to override node24 on github runner

[peterzhuamazon]

Description

Using node20 to override node24 on github runner, support multiple v5 action upgrades by github.
https://github.com/opensearch-project/job-scheduler/actions/runs/17776011934/job/50523797065?pr=818

Issues Resolved

Resolves: #5178

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.56%. Comparing base (a765213) to head (809552c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5723   +/-   ##
=======================================
  Coverage   96.56%   96.56%           
=======================================
  Files         404      404           
  Lines       18440    18440           
=======================================
  Hits        17806    17806           
  Misses        634      634           

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