Update the internal setup-node pin to v6.3.0 (#85)

## Summary
- update the internal `actions/setup-node` pin in `action.yml` from the
v4.4.0 commit to the v6.3.0 commit
- change `node-version: "20"` to `"24"`
- address the Node 20 deprecation warning caused by the action
dependency itself

## Verification
- `pnpm install --frozen-lockfile`
- `pnpm run check`
- `pnpm run build`

Closes #84

---------

Co-authored-by: Michael Bolin <[email protected]>

action.yml

@@ -123,9 +123,9 @@ runs:
     - name: Ensure Node.js available
       # Pin to a commit hash because some repositories require it:
       # https://github.com/openai/codex-action/issues/43
-      uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
+      uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
       with:
-        node-version: "20"
+        node-version: "24"
 
     - name: Check repository write access
       env: