Coding Standards: Move wp-admin/custom-background.php to wp-admin/includes/class-custom-background.php

This renames the file containing the Custom_Background class to conform to the coding standards.

This commit also includes:

- A new custom-background.php that includes the new file, for anyone that may've been including the file directly.
- Replaces a reference to the old filename with the new filename.

See #47632.

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