ROU-13006: Add Slack notification to the release workflow - #1301

[os-davidlourenco]

This PR is for adding a Slack notification to the release workflow so the team is notified whenever a release PR is created.

What was happening

• The release workflow merged the rc branch directly into main with no review step and no notification that a release was in progress.
• The rc branch was deleted automatically at the end of the process.

What was done

• Replaced the direct merge step with a step that creates a release PR from the rc branch into main using the GitHub CLI (skipped when the rc branch is already merged or an open PR already exists).
• Added a Slack notification step that posts to the channel when the release PR is created.
• Added a new required secret OSUI_SLACK_WEBHOOK_URL to the reusable workflow, so each repository using this workflow can provide its own Slack app and webhook.
• Removed the delete-rc-branch job and input, since deleting the rc branch would automatically close the newly created release PR.
• Updated the caller workflow to forward the new secret and grant the pull-requests write permission.

Test Steps

1. Add the OSUI_SLACK_WEBHOOK_URL secret to the repository settings.
2. Run the Release workflow with a valid rc version.
3. Confirm a PR from the rc branch into main is created and the direct merge no longer happens.
4. Confirm the Slack channel receives the notification referencing the new PR.

Screenshots

(prefer animated gif)

Checklist

• tested locally
• documented the code
• clean all warnings and errors of eslint
• requires changes in OutSystems (if so, provide a module with changes)
• requires new sample page in OutSystems (if so, provide a module with changes)

[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