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/Common/Helper/CellTypeHelperTest.php

@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 class CellTypeHelperTest extends TestCase
 {
     /**
-     * @return array
+     * @return void
      */
     public function testIsEmpty()
     {
@@ -27,7 +27,7 @@ class CellTypeHelperTest extends TestCase
     }
 
     /**
-     * @return array
+     * @return void
      */
     public function testIsNonEmptyString()
     {
@@ -44,7 +44,7 @@ class CellTypeHelperTest extends TestCase
     }
 
     /**
-     * @return array
+     * @return void
      */
     public function testIsNumeric()
     {
@@ -66,7 +66,7 @@ class CellTypeHelperTest extends TestCase
     }
 
     /**
-     * @return array
+     * @return void
      */
     public function testIsBoolean()
     {