Switch to new PHPCSStandards/PHP_CodeSniffer package

[mindctrl]

The Squizlabs/PHP_CodeSniffer package has been deprecated. It has been forked and will be maintained under a new package at PHPCSStandards/PHP_CodeSniffer.

Reference: The Future of PHP_CodeSniffer · Issue #3932 · squizlabs/PHP_CodeSniffer

Acceptance Criteria

• Existing squizlabs package is removed and replaced with the new one.
• Existing phpcs tests continue to pass. If failures happen after upgrading, the changes needed to make it pass should happen in the same PR.

[ChrisWiegman]

While the link here indicates that we didn't need to actually change the package name (they managed to keep the same namespace on Packagist), I did update us to versions of WPCS and phpcs that followed the changes from the original change in #1894