BUILD-10835 Remove slack_notify reusable workflow

[hedinasr]

BUILD-10835 This change removes the slack_notify reusable workflow from ci-github-actions.

The workflow triggered on check_suite: completed and forwarded results to Slack. It is dropped as part of the BUILD-10835 CI consolidation so this repo no longer ships or maintains that entry point.

[hashicorp-vault-sonar-prod]

BUILD-10835

[sonar-review-alpha]

Summary

This PR removes .github/workflows/slack_notify.yml, a reusable workflow that automatically sent Slack notifications when check suites completed. The workflow was part of ci-github-actions' public API surface for external repositories to use, so removal is a breaking change for any downstream consumers. This aligns with the BUILD-10835 CI consolidation effort to streamline the repo's maintained workflows.

The change is straightforward: a single file deletion. No other workflows reference slack_notify.yml, and the Slack notification logic appears to have been consolidated elsewhere (or deprecated entirely as part of this initiative).

What reviewers should know

What to verify:

• Check that no internal SonarSource workflows still reference this workflow (search existing workflows for slack_notify)
• Confirm downstream teams have been notified about the breaking change (check linked JIRA BUILD-10835 or related communications)
• Verify the squad-eng-xp-github Slack channel is receiving notifications through an alternative mechanism if still needed

Scope: File deletion only—no changes to other workflows or code. The workflow was self-contained with no internal dependencies.

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[sonar-review-alpha]

LGTM! ✅

Clean, minimal change. The runner label swap in slack_notify.yml and the corresponding actionlint.yaml allowlist update are consistent and complete — nothing was missed.

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[Copilot]

Pull request overview

Removes the slack_notify reusable GitHub Actions workflow from this repository as part of BUILD-10835 CI consolidation, so this repo no longer provides the check_suite: completed → Slack forwarding entry point.

Changes:

• Deleted .github/workflows/slack_notify.yml, removing the check_suite trigger and Slack notification job.
• Eliminated the dependency on SonarSource/gh-action_slack-notify from this repo (as the workflow is no longer shipped).

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues
0 New dependency risks

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud