Composer: update dependencies

This commit:
* Removes the dependency on the `dealerdirect/phpcodesniffer-composer-installer` plugin.
    This is already a dependency of the `doctrine/coding-standard` package and having it explicitly in the `composer.json` file is just inviting version constraint conflicts.
* Updates all dependencies.
    Updates:
    - `dealerdirect/phpcodesniffer-composer-installer` from version `0.5.0` to `0.7.1`
        The most notable change here is that this adds support for Composer 2.x as well as support for PHP 8.0.
    - `doctrine/coding-standard` from version `7.0.2` to `9.0.0`
    - `phpstan/phpdoc-parser` from version `0.4.3` to `0.5.5`
    - `slevomat/coding-standard` from version `6.1.5` to `7.0.13`
    - `squizlabs/php_codesniffer` from version `3.5.4` to `3.6.0`
        The most notable change here is that this adds support for PHP 8.0.

Refs:
* https://github.com/Dealerdirect/phpcodesniffer-composer-installer/releases
* https://github.com/doctrine/coding-standard/releases
* https://github.com/phpstan/phpdoc-parser/releases
* https://github.com/slevomat/coding-standard/releases
* https://github.com/squizlabs/php_codesniffer/releases

Author: jrfnl

composer.json

@@ -22,8 +22,7 @@
   },
   "require": {
     "php": "^7.2",
-    "dealerdirect/phpcodesniffer-composer-installer": "^0.5.0",
-    "doctrine/coding-standard": "^7.0"
+    "doctrine/coding-standard": "^9.0"
   },
   "config": {
     "sort-packages": true