This renames the file containing the `wpdb` class to conform to the coding standards.
This commit also includes:
* A new `wp-db.php` that loads the new file, for anyone that may have been including the file directly.
* Replacing references to the old filename with the new filename.
Fixes#56268. See #55647.
git-svn-id: https://develop.svn.wordpress.org/trunk@53749 602fd350-edb4-49c9-b593-d223f7449a82