Bump minimum PHP and WordPress versions

[GaryJones]

The minimum PHP requirement for the current release (0.5) is not clear - we can assume PHP 5.2. This should be bumped to PHP 7.1. This allows type declarations and strictly typed user-land code, which can help with the more strictly typed code from native PHP 8.1 functions.

The minimum WP requirement for the current release (0.5) is WP 4.5. This should be bumped to WP 5.5 in the first instance.

These changes should be reflected in composer.json, readme headers, workflows, changelog and other documentation.