Rename the `alerts-general` Slack channel to `feed_alerts-general`

related to: gitlab-com/support/support-team-meta#2244 (closed)

The slackline project has a CI / CD environment variable for the default channel to post to. This PRODUCTION_ENV_YAML only needs to be updated so the SLACK_DEFAULT_CHANNEL: is feeds_alerts-general and the application redeployed via the pipeline.