Fix GitHub Actions permissions for PR preview workflow

[ajeetraina]

This PR fixes the GitHub Actions workflow that was failing with the error: "The process '/usr/bin/git' failed with exit code 128".

Changes made:

1. Added top-level permissions to the workflow:

   permissions:
     contents: write
     pull-requests: write

2. Replaced personal_token with github_token in the actions-gh-pages step:

   github_token: ${{ secrets.GITHUB_TOKEN }}

3. Downgraded checkout action from v3 to v2 for the cleanup job, which has fewer issues with permissions

These changes will allow the GitHub Actions workflow to properly access the repository and make commits to the gh-pages branch. The issue occurs because GitHub Actions needs explicit permissions to write to the repository.

Ref: actions/checkout#417

[github-actions]

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