workflows: drop old node, add 24.x, only lint once

[MaxLeiter]

• Dropped Node 16.x and 18.x, added 24.x
• Split into two jobs: lint (runs once on Node 22) and test. before we were running lint on every version
• Test job runs mocha + nyc directly nstead of also running lint

[Copilot]

Pull request overview

Updates the CI test workflow to better align with the project’s supported Node.js versions while reducing redundant work in GitHub Actions.

Changes:

• Dropped Node.js 16.x/18.x from the test matrix and added 24.x.
• Split CI into a dedicated lint job (runs once on Node 22.x) and a test matrix job.
• Updated the test job to run yarn coverage (nyc+mocha) instead of yarn test (which also runs lint).

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