Add Slack notification for potential regression issues

[bhoradc]

Motivation and Context

Add a Slack notification when a potential regression issue is filed on the repo. This gives the team immediate visibility into regression reports without having to manually monitor GitHub issues.

Modifications

Added a Notify Slack step to the existing issue-regression-labeler workflow to notify the team channel when a potential regression issue is opened.

• Only triggers on opened events (not edits) to avoid duplicate notifications
• Uses CI_SLACK_WEBHOOK_URL repo secret (already configured)

Testing

Validated end-to-end in a private test repo.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[github-actions]

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