Update Web UI Deployment doc for LTS 2021

The HOWTO: Deploy Web UI Within a Nuxeo Server documentation for LTS 2021 specifies to have the following require directive in the contribution .xml:

<require>org.nuxeo.web.ui.resources</require>

As of LTS 2021, this is no longer valid as this component no longer exists. Attempting server startup with this require in the contribution .xml results in deployment failures.

All that is needed now is a require in the deployment-fragment .xml (detailed in Web UI Customization Tutorial). The web-ui-deployment documentation should be updated to reflect current requirements.