Change YAML config to PHP

[OndraM]

Unfortunately, another breaking change introduced by easy-coding-standard (ECS)...

As part of deprecated-packages/symplify#2008, they switched from easy-coding-standard.yaml to config in PHP file ecs.php. YAML was deprecated in ECS 8.1.21 (the deprecation always comes with sleep 3 to force anyone to upgrade 🤦 ). It was removed in ECS 9.

TODO

• Update README
• Add upgrade howto