Fixed php deprecation warning

[girishpanchal30]

Summary

Replaced deprecated variable interpolation ${site_slug} with the recommended syntax {$site_slug} to ensure compatibility with PHP 8.2 and above.

Will affect visual aspect of the product

NO

Test instructions

• Run the following sample command:
• sudo -u php-fpm-user -i -- wp search-replace "staging.garriganenterprises.com/bear" "www.garriganenterprises.com" --all-tables --precise --path=/usr/share/nginx/html/www
• Ensure that no PHP deprecation warnings (such as ${var} usage) are displayed during execution.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #436.

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