Upgrade to checkout@v4

[pquentin]

Validation currently fails, see https://github.com/elastic/elasticsearch-specification/actions/runs/9093977704/job/25087280424

[pquentin]

I added a dummy file to trigger CI, which happened here: https://github.com/elastic/elasticsearch-specification/actions/runs/9123956086/job/25087262219. And the checkout worked, even though the dummy file broke validation itself.

[github-actions]

The backport to 7.17 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-7.17 7.17
# Navigate to the new working tree
cd .worktrees/backport-7.17
# Create a new branch
git switch --create backport-2557-to-7.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 f96234e26b0ed2482580c4e9f09455a56b5f7347
# Push it to GitHub
git push --set-upstream origin backport-2557-to-7.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.17

Then, create a pull request where the base branch is 7.17 and the compare/head branch is backport-2557-to-7.17.