BREAKING CHANGE: Remove ecs-8.x.php files to simplify configuration

[OndraM]

I tend to believe we no longer need these files with extra config for specific PHP version:

• There aren't any check in them
• Fixers which depends on some PHP version (eg. they require PHP 8.1 features) have version detection build inside them and they don't run if they detect unsupported PHP version
• In case we would need such feature to make PHP version dependent config, we can do it directly in the ecs.php file (if (PHP_VERSION >= xxx) { ... })

So because of the near-to-zero value and to make the configuration simpler I suggest removing these extra files.