chore: rollback devDep npm-run-all2 to v8 for Node 20

[mark-wiemer]

PR Checklist

• Addresses an existing open issue: fixes 🛠️ Repo: npm i with Node 22.21.1 gives EBADENGINE for npm-run-all2 #6038
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

v9 of this devDep has an undocumented breaking change of only supporting Node 22.22+

Ref #6019 and bcomnes/npm-run-all2#231

[JoshuaKGoldberg]

I re-triggered the tests (#6042). Once this passes, ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.08%. Comparing base (a8ffaa9) to head (872a1e7).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6040   +/-   ##
=======================================
  Coverage   78.08%   78.08%           
=======================================
  Files          63       63           
  Lines        4581     4581           
  Branches      978      978           
=======================================
  Hits         3577     3577           
  Misses       1004     1004           

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[mark-wiemer]

Known test failures (#6036 and #6037 ), merging :)