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

src/Spout/Writer/Common/Entity/Sheet.php

@@ -10,7 +10,7 @@ use Box\Spout\Writer\Common\Manager\SheetManager;
  */
 class Sheet
 {
-    const DEFAULT_SHEET_NAME_PREFIX = 'Sheet';
+    public const DEFAULT_SHEET_NAME_PREFIX = 'Sheet';
 
     /** @var int Index of the sheet, based on order in the workbook (zero-based) */
     private $index;