ci: Stop running CI for Node.js 18

[Mrtenz]

To reduce the number of concurrent jobs, we can stop running CI for Node.js 18 (since it's EOL in a few days anyway).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (d75fa4d) to head (7fbe7c6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3353   +/-   ##
=======================================
  Coverage   98.03%   98.03%           
=======================================
  Files         402      402           
  Lines       11110    11110           
  Branches     1754     1754           
=======================================
  Hits        10892    10892           
  Misses        218      218           

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