Allow to not post incidents to the group Slack channel

What does this MR do and why?

In the case of failing dev pipelines, if an incident gets the infrastructure label set, then the infrastructure group is detected, and we send the notification to the Slack channel defined for the group (i.e. #infrastructure-lounge before this MR).

The thing is that we shouldn't post bot message in #infrastructure-lounge but only to the default channel (i.e. #master-broken-mirrors) defined in the DevMasterBranch pipeline failure config class.

I initially wanted to remove the channel for the infrastructure group with gitlab-com/www-gitlab-com!134242 (closed) but linting was complaining, so instead I opened this MR to allow pipeline failure configs to disable sending incidents to the detected group.

That was reported by @marin at https://gitlab.slack.com/archives/C04RC5QTGBT/p1714637448630389.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review