Suppress npm update notifier - #21363

Description

Suppress the npm update notifier by setting NPM_CONFIG_UPDATE_NOTIFIER=false for frontend-maven-plugin executions.

Without this, npm can periodically print its update notification to stderr, which Maven reports as errors:

[ERROR] npm notice
[ERROR] npm notice New major version of npm available! 11.16.0 -> 12.0.2
[ERROR] npm notice Changelog: https://github.com/npm/cli/releases/tag/v12.0.2
[ERROR] npm notice To update run: npm install -g npm@12.0.2
[ERROR] npm notice

These messages do not indicate a build failure but make the build output unnecessarily look like it contains errors.

Testing

Remove npm's update-notifier timestamp to force it to check for updates again:

rm ~/.npm/_update-notifier-last-checked

Then run a new build.

The npm update notice should no longer be shown. The _update-notifier-last-checked file should also remain absent, confirming that the update notifier was disabled rather than merely suppressed because npm had recently checked for updates.