CI: bump actions/checkout v4 → v6 across workflows

[juangaitanv]

actions/checkout@v4 is deprecated. This bumps all remaining usages to @v6 (current; v6.0.3) across the repo's workflows:

• test.yml
• release.yml (×4)
• release-binaries.yml
• npm-publish.yml
• update_docs..yml

Addresses review feedback from @leenk7991 on #110 (discussion_r3412467742). The staging-contract.yml workflow added in #110 was already bumped on that branch.

[cursor]

No actionable findings.

What I verified:

• Compared origin/main (aea9287576bf42c201b7cbc19c36bba1dceffd50) against PR head (a5cba81525cfaa9e6ac6368c7ff4591056257eb0); the diff only changes the eight actions/checkout call sites from v4 to v6 across the five workflow files.
• Confirmed upstream actions/checkout@v6 exists and currently resolves to the v6.0.3 line. Its metadata uses node24, and all changed jobs use GitHub-hosted runner labels rather than self-hosted runners with unknown runner versions.
• Checked the v6 credential-behavior change: these workflows do not run authenticated git fetch/git push or read checkout credentials from .git/config after checkout. The Docker-based maturin job does not rely on checkout credentials, and the docs sync action receives explicit tokens.
• Checked for submodules/LFS and checkout-dependent repo config; none were present in this repo.
• git diff --check produced no whitespace or patch-format issues.
• Live PR checks already show Test, sdist, and most Native Binary Release matrix jobs passing with checkout@v6; remaining long-running CI jobs were still in progress at review time, so merge should continue to respect required-check gating.

  

_{Sent by Cursor Automation: pr-flow}

[Ibrahimrahhal]

lets use a commit and have a comment so we know which version