ci: upgrade actions to Node.js 24

[Willxup]

Summary

• Upgrade every CI and release action reported by GitHub as targeting deprecated Node.js 20 to an official action release that declares the Node.js 24 runtime.
• Keep the installed toolchains aligned at Node.js 24.14.0 and Go 1.26.2 across CI and Docker, and add a release-contract test that locks the complete approved action list.

Verification

• make check
• make release-check GITLEAKS=/Users/will/Projects/flowlens/.flowlens-dev/tools/bin/gitleaks
• make frontend-e2e not required because UI behavior is unchanged
• Public API documentation not applicable because the API is unchanged
• No real configuration, credential, Cookie, database, backup, log, or private address added
• Commits include Signed-off-by (DCO)

Compatibility

No configuration, database, API, or container interface changes. GitHub-hosted action runners now use Node.js 24-compatible action majors; the application build remains pinned to Node.js 24.14.0 and Go 1.26.2.