ROU-13006: Add Slack notification to the release workflow

[os-davidlourenco]

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

What was happening

• The release workflow merged the rc branch directly into main, with no visibility for the team when a release was in progress.
• There was no way for each repository reusing the release workflow to configure its own Slack app and webhook.

What was done

• Replaced the direct rc-into-main merge with a step that creates a release PR, skipping creation when the branches are identical or a PR already exists.
• Added a Slack notification step that posts the release PR reference when a new PR is created.
• Added OSUI_SLACK_WEBHOOK_URL as a required workflow secret so each repository using this reusable workflow can provide its own Slack app and webhook.
• Removed the delete-rc-branch input and job, since deleting the rc branch would auto-close the open release PR.
• Granted pull-requests write permission to the jobs that create the release PR.

Test Steps

1. Configure the OSUI_SLACK_WEBHOOK_URL secret in the repository settings.
2. Run the Release workflow with a version that has an existing rc branch.
3. Confirm a PR from the rc branch into main is created and the Slack channel receives the notification.
4. Re-run the workflow and confirm no duplicate PR is created and no duplicate notification is sent.

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

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