v55 - PHP 8.1

This update changes the version of PHP used from 8.0 (which is past its end of live) to 8.1. It also includes some documentation updates that do not affect the application itself.

What's Changed

• Add how-to document for applying Pantheon updates by @matt-bernhardt in #137
• Upgrade PHP to 8.1 by @matt-bernhardt in #134

Full Changelog: v54...v55