License Issues around slack.svg

### Issue description

See https://github.com/apache/grails-core/pull/15792#discussion_r3501902907 for history.

While we created custom chat icons for slack, we still name those icons slack.svg. We should consider renaming them to something more generic like chat.svg and then have the generated apps say "Chat with us" or something so we don't refer directly to an external tool.

Start with the linked pull request discussion and search the repository for slack.svg and the generated-app wording. Rename the custom icon to a generic name and update generated apps to use generic chat text, confirming that no remaining references identify Slack.

Written by the indexing model from the issue text.