Specify global search alerts channel

What does this MR do and why?

Uses designated global search alerts channel for alert notifications. See related slack thread: https://gitlab.slack.com/archives/CJHPLRTRD/p1721697938642299 (internal)

Expected impact & dry-runs

Messages like these https://gitlab.slack.com/archives/C3TMLK465/p1721685409868439 will go to #g_global_search_alerts instead of #g_global_search

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