Bump xmldom and plist in /lab/webapp

Removes [xmldom](https://github.com/xmldom/xmldom). It's no longer used after updating ancestor dependency [plist](https://github.com/TooTallNate/node-plist). These dependencies need to be updated together.


Removes `xmldom`

Updates `plist` from 3.0.1 to 3.0.6
- [Release notes](https://github.com/TooTallNate/node-plist/releases)
- [Changelog](https://github.com/TooTallNate/plist.js/blob/master/History.md)
- [Commits](https://github.com/TooTallNate/node-plist/commits)

---
updated-dependencies:
- dependency-name: xmldom
  dependency-type: indirect
- dependency-name: plist
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>