ci: bump GitHub Action pins to node24-compatible majors

[deorus]

Bumps GitHub Action pins to majors that ship with the node24 runtime, ahead of GitHub's node20 deprecation.

• actions/checkout v4 → v6
• googleapis/release-please-action v4 → v5

Tracking: https://github.com/Doist/platform-backlog/issues/1385

[doistbot]

This PR updates Bump GitHub Action pins to node24-compatible majors. No issues were flagged in the reviewed diff.

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[nats12]

@deorus looks good, we may need to rename the commit so we get a full CI pass due to semantic title checks.

[engfragui]

@nats12 I was about to merge this anyway (with the non-standard commit message) but it doesn't seem like there's a way to force that (for this repo specifically), probably due to permissions? 🤔
Either way, I'll make sure that the "Validate Title" check passes so that we can merge it.