Replace ``mattermost-support`` GitLab account with a link to the Mattermost support portal

At https://handbook.gitlab.com/handbook/support/workflows/mattermost/#escalating-to-the-mattermost-team, the GitLab mattermost-support account can be removed and instead we can link to the Mattermost support portal https://mattermost.com/support/.

The GitLab mattermost-support account is no longer a valid channel and we'd want to remove that support group tag. We'd instead want the support portal to be used to submit issues.

cc @dorrino