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

[RanVaknin]

Can we add a part of the body of the Github issue as well?
Maybe only the first 100 words or something so we can assess the issue without clicking through to Github.

[bhoradc]

That's a good idea - I considered it too. My primary intention of this notification is to prompt us to triage the issue as soon as possible, so clicking through to the issue is the intended action. Then looking at past regression issues, the titles were quite descriptive, so I felt adding body content could make the channel messy. Additonally, it also may contain markdown, code blocks, stack traces that may not render cleanly in Slack. Happy to revisit if we find the title alone isn't enough, WDYT?