Update slack_notifications.yml as part of instrumentation generator

[kaylareopelle]

Most of the time, when we create instrumentation for a gem, we'll want to stay informed about the latest releases. The slack_notifications.yml workflow helps us do that. Let's make updating this file part of the generator task.

Another implementation option: #2553

[workato-integration]

https://new-relic.atlassian.net/browse/NR-310167