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/XLSX/Manager/WorksheetManager.php

@@ -32,9 +32,9 @@ class WorksheetManager implements WorksheetManagerInterface
      * @see https://support.office.com/en-us/article/Excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3 [Excel 2010]
      * @see https://support.office.com/en-us/article/Excel-specifications-and-limits-ca36e2dc-1f09-4620-b726-67c00b05040f [Excel 2013/2016]
      */
-    const MAX_CHARACTERS_PER_CELL = 32767;
+    public const MAX_CHARACTERS_PER_CELL = 32767;
 
-    const SHEET_XML_FILE_HEADER = <<<'EOD'
+    public const SHEET_XML_FILE_HEADER = <<<'EOD'
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 <worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships">
 EOD;