PHPCS/Composer: update PHPCompatibility

Composer:
* `wimg/php-compatibility` has been abandoned for over a year. Use `phpcompatibility/php-compatibility` instead.
* Use the latest version of PHPCompatibility.
    You were missing out on a lot of new checks, including the checks to make sure your code is compatible with the upcoming PHP 7.4.
* As of PHPCompatibility 8.0, the directory layout of the PHPCompatibility standard has been updated to work correctly with Composer, so no need for the custom scripts moving the files anymore.
* Add the DealerDirect Composer PHPCS plugin.
    This plugin will handle setting the PHPCS `installed_paths` automatically.
    This also allows for referencing the ruleset by name instead of via the path in the ruleset, which is generally more stable.

PHPCS ruleset:
* Rename the ruleset to `phpcs.xml.dist` which will allow PHPCS to automatically pick up on it.
    No need to pass the `--standard=...` command-line argument anymore.
* Check for cross-version compatibility for the PHP versions officially supported.
    According to the `composer.json` file, this code should be compatible with PHP 5.5 and above.
    PHPCompatibility _was_ checking against PHP 5.5 up to PHP 7.0.
    The new `testVersion` actually checks against PHP 5.5 up to the latest version (7.4 at this moment).

Refs:
* https://github.com/PHPCompatibility/PHPCompatibility#sniffing-your-code-for-compatibility-with-specific-php-versions
* https://github.com/PHPCompatibility/PHPCompatibility/releases/
* https://github.com/Dealerdirect/phpcodesniffer-composer-installer

Suggestion:
* You may also want to update the Symfony standard dependency which is currently at version `3.11.0`
    Ref: https://github.com/djoos/Symfony-coding-standard/blob/master/UPGRADE-3.0.md
    Note: the ruleset name has been changed in version 3 from `Symfony2` to `Symfony`.
* And consider updating (or removing) the PHP_CodeSniffer dependency, of which the latest release is version `3.5.6`
    I'd suggest removing it as it is not _your_ dependency, but a dependency of the PHPCompatibility and the Symfony coding standards, so let those dependencies manage the version rather than doing that yourself.
    Ref: https://github.com/squizlabs/php_codesniffer/releases

composer.json

@@ -23,18 +23,9 @@
     "require-dev": {
         "squizlabs/php_codesniffer": "^2.5",
         "escapestudios/symfony2-coding-standard": "^2.9",
-        "wimg/php-compatibility": "^7.0"
+        "phpcompatibility/php-compatibility": "^9.0",
+        "dealerdirect/phpcodesniffer-composer-installer": "^0.7"
     },
-    "scripts": {
-        "default-scripts": [
-            "rm -rf vendor/squizlabs/php_codesniffer/CodeSniffer/Standards/PHPCompatibility; cp -rp vendor/wimg/php-compatibility vendor/squizlabs/php_codesniffer/CodeSniffer/Standards/PHPCompatibility"
-        ],
-        "post-install-cmd": [
-            "@default-scripts"
-        ],
-        "post-update-cmd": [
-            "@default-scripts"
-        ]
-    },
-    "minimum-stability": "dev"
+    "minimum-stability": "dev",
+    "prefer-stable": true
 }

ruleset.xml → phpcs.xml.dist

@@ -5,10 +5,10 @@
     <exclude-pattern>vendor/</exclude-pattern>
     <exclude-pattern>Resources/</exclude-pattern>
 
-    <rule ref="vendor/escapestudios/symfony2-coding-standard/Symfony2"/>
-    <rule ref="vendor/squizlabs/php_codesniffer/CodeSniffer/Standards/PHPCompatibility" />
+    <rule ref="Symfony2"/>
+    <rule ref="PHPCompatibility"/>
 
-    <config name="testVersion" value="5.5-7.0"/>
+    <config name="testVersion" value="5.5-"/>
 
     <rule ref="PEAR.Functions.ValidDefaultValue.NotAtEnd">
         <severity>0</severity>