Fix raw sql table post meta declaration

[ogorzalka]

Description of the Change

In this pull request, I have made a minor modification to how the postmeta table is referenced. Previously, the postmeta table was directly referenced by its name (e.g., wp_postmeta). I have modified this reference to use the $wpdb->postmeta variable, which is a standard practice in WordPress to ensure better compatibility with custom WordPress installations and multi-site environments.

Advantages:

• Improved compatibility with custom WordPress installations.
• Better support for multi-site environments.
• Alignment with WordPress best practices and standards.

How to test the Change

1. Install the plugin with custom database prefix.
2. Set up some redirections using the plugin or import redirection with csv file.
3. Verify that the redirections are working correctly.

Changelog Entry

Changed - Improved reference to the postmeta table for better WordPress compatibility.

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my change.
• [x] All new and existing tests pass.