build: upgrade workflow actions for node 24

[gpbl]

GitHub is warning that several workflow actions in this repo still run on the deprecated Node 20 JavaScript action runtime. This updates the workflow action majors so the repo is on supported Node 24-compatible releases before GitHub flips the default runtime.

What Changed

• upgraded actions/checkout to v6, actions/setup-node to v6, and pnpm/action-setup to v5 in the main CI workflow
• applied the same action upgrades to the release and website workflows so the repo is consistent
• removed stale always-auth and EXPECTED_* workflow env configuration from release.yml now that the current release scripts do not use it

Review Notes

• the intent here is workflow-runtime maintenance only; package build, test, and release behavior should stay the same
• actions/setup-node@v6 removed the deprecated always-auth input, so this PR also drops that no-op config from the release workflow
• I could not run actionlint locally because it is not installed in this repo, so the main validation for syntax will come from GitHub Actions on the PR

[changeset-bot]

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Latest commit: 3bfb605

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