Add workflow to clean stale PRs

[gpunto]

Goal

Introduce weekly check for cleaning stale PRs

Implementation

Add workflow invoking the corresponding one in android-ci-actions.

Testing

Open a PR, let it go stale, verify that is properly addressed.

Checklist

• Issue linked (if any)
• Tests/docs updated
• I have signed the Stream CLA (required for external contributors)

[github-actions]

PR checklist ✅

All required conditions are satisfied:

• Title length is OK (or ignored by label).
• At least one pr: label exists.
• Sections ### Goal, ### Implementation, and ### Testing are filled.

🎉 Great job! This PR is ready for review.

[Copilot]

Pull Request Overview

This PR introduces an automated workflow to clean up stale pull requests in the repository. The workflow runs weekly on Mondays and can also be triggered manually.

• Adds a GitHub Actions workflow that schedules stale PR cleanup every Monday at 7:00 AM UTC
• Leverages an existing reusable workflow from the GetStream/android-ci-actions repository
• Configures appropriate permissions for pull request and issue management

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