chore: bump action runtime to Node 24 (fixes #448)

[MukundaKatta]

Summary

Fixes #448.

GitHub will force all Node-20 JavaScript actions onto Node 24 on June 2nd 2026 and surfaces a deprecation annotation on every run in the meantime. Every `action-hosting-deploy` invocation today emits:

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: FirebaseExtended/action-hosting-deploy@…

This PR bumps `runs.using` from `node20` → `node24` in `action.yml` so the warning goes away and we're on the other side of the forced cutover. That's the only line that changes.

Scope (deliberately minimal)

• Single-line change in `action.yml`.
• `npm run build` rebuilds cleanly and produces a byte-identical `bin/action.min.js` (the runtime declaration doesn't affect microbundle's output, which is already `--target node`). The `check-build.yml` workflow's `git diff --exit-code` on `*.min.js` should pass.
• `npm run format:check` clean.
• `npm run test` — 13 passed, 0 failed.
• No source, dependency, or API changes.
• No consumer-side change required; workflows using `FirebaseExtended/action-hosting-deploy@v0` keep working.

Verification

```
$ npm run format:check
(clean)

$ npm run build
Build "firebase-hosting-preview-action" to bin:
177 kB: action.min.js.gz
126 kB: action.min.js.br

$ git diff --stat -- "*.min.js"
(empty)

$ npm run test
Test Suites: 3 passed, 3 total
Tests: 13 passed, 13 total
```

Notes for the reviewer

• Node 24 is the version GitHub will force runtime to on June 2nd 2026 (changelog). The currently-in-flight `ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION` escape hatch is explicitly temporary, so moving now is the cleanest path.
• Issue [BUG] Support Node 24 #440 ("Support Node 24") is the same ask from a bug angle — this PR closes both.
• CLA: I'll sign the Google CLA when the check runs if I haven't already.

Checklist

• `npm run format:check`
• `npm run build` — bundle unchanged
• `npm run test` — 13/13 pass
• No consumer workflow breakage (action input/output surface untouched)

[gemini-code-assist]

Code Review

This pull request updates the GitHub Action runtime from Node 20 to Node 24. Feedback suggests aligning the development environment, specifically the TypeScript configuration and package dependencies, with the new Node 24 target to ensure consistency across environments.

[gemini-code-assist]

While bumping the runtime to Node 24 addresses the deprecation warning, the project's development configuration remains significantly outdated. Specifically, package.json still references @tsconfig/node12 and older versions of development dependencies. To ensure consistent behavior between development, testing, and production environments, and to leverage modern Node.js features or optimizations, it is recommended to update the TypeScript configuration and development environment to align with the Node 24 target.

[jhuleatt]

Thank you @MukundaKatta!