fix: Upgrade GitHub Actions to latest versions

Merge #3933

## Summary

Upgrade GitHub Actions to their latest versions for improved features, bug fixes, and security updates.

## Changes

| Action | Old Version(s) | New Version | Release | Files |
|--------|---------------|-------------|---------|-------|
| `google-github-actions/auth` | [`v2`](https://github.com/google-github-actions/auth/releases/tag/v2) | [`v3`](https://github.com/google-github-actions/auth/releases/tag/v3) | [Release](https://github.com/google-github-actions/auth/releases/tag/v3) | discussion_answering.yml, upload-adk-docs-to-vertex-ai-search.yml |

## Why upgrade?

Keeping GitHub Actions up to date ensures:
- **Security**: Latest security patches and fixes
- **Features**: Access to new functionality and improvements
- **Compatibility**: Better support for current GitHub features
- **Performance**: Optimizations and efficiency improvements

### Security Note

Actions that were previously pinned to commit SHAs remain pinned to SHAs (updated to the latest release SHA) to maintain the security benefits of immutable references.

### Testing

These changes only affect CI/CD workflow configurations and should not impact application functionality. The workflows should be tested by running them on a branch before merging.

Co-authored-by: Xiang (Sean) Zhou <[email protected]>
COPYBARA_INTEGRATE_REVIEW=#3933 from salmanmkc:upgrade-github-actions-node24-general 142ef77
PiperOrigin-RevId: 845804209

Author: 2 people

.github/workflows/discussion_answering.yml

@@ -24,7 +24,7 @@ jobs:
 
       - name: Authenticate to Google Cloud
         id: auth
-        uses: 'google-github-actions/auth@v2'
+        uses: 'google-github-actions/auth@v3'
         with:
           credentials_json: '${{ secrets.ADK_GCP_SA_KEY }}'
 

.github/workflows/upload-adk-docs-to-vertex-ai-search.yml

@@ -28,7 +28,7 @@ jobs:
 
       - name: Authenticate to Google Cloud
         id: auth
-        uses: 'google-github-actions/auth@v2'
+        uses: 'google-github-actions/auth@v3'
         with:
           credentials_json: '${{ secrets.ADK_GCP_SA_KEY }}'