Clear CI warnings

[gokaygurcan]

There're 3 of them at this moment:

• Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v2, actions/upload-artifact@v2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
• The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2, actions/upload-artifact@v2. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/
• The following artifacts were uploaded using a version of actions/upload-artifact that is scheduled for deprecation: "ab-results".
  Please update your workflow to use v4 of the artifact actions.
  Learn more: https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/

Nothing show stopper, but would be nice to clear them at some point.

[hiSandog]

Since these versions are now several generations behind, updating directly to actions/checkout@v4 and actions/upload-artifact@v4 would avoid doing the migration twice. The artifact step deserves a quick check for v4's immutable artifact-name behavior and retention settings, especially if matrix jobs currently upload to the same ab-results name. A workflow-only PR can verify the clean warning state without changing the project content.