chart: update contact info in chart.yaml

General MR

Summary

Change the primary maintainer email from Andrew to a link to the public Big Bang community Slack. We don't have a specific channel over there but if someone were to post about bbctl in that chat, someone would surely link it back to us on Mattermost, and it's unclear if linking to the IL4 mattermost will work since the server requires a dedicated invite.

This is in lieu of providing a group email.

Relevant logs/screenshots

N/A

Linked Issue

#266 (closed)

Upgrade Notices

N/A