Upgrade PHP and dev dependencies

/!\ Removed PHP 7.2 support /!\

- PHPUnit 8 => 9 (+ fix the tests)
- PHP-CS-Fixer 2 => 3 (+ fix the code)
- Introduced PHP stan

tests/Spout/Writer/ODS/SheetTest.php

@@ -95,7 +95,7 @@ class SheetTest extends TestCase
     /**
      * @param string $fileName
      * @param string $sheetName
-     * @return Sheet
+     * @return void
      */
     private function writeDataAndReturnSheetWithCustomName($fileName, $sheetName)
     {