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)
     {

tests/Spout/Writer/ODS/WriterTest.php

@@ -132,7 +132,7 @@ class WriterTest extends TestCase
             $writer->addRows($dataRows);
             $this->fail('Exception should have been thrown');
         } catch (SpoutException $e) {
-            $this->assertFileNotExists($fileName, 'Output file should have been deleted');
+            $this->assertFileDoesNotExist($fileName, 'Output file should have been deleted');
 
             $numFiles = iterator_count(new \FilesystemIterator($tempFolderPath, \FilesystemIterator::SKIP_DOTS));
             $this->assertEquals(0, $numFiles, 'All temp files should have been deleted');

tests/Spout/Writer/XLSX/WriterTest.php

@@ -163,7 +163,7 @@ class WriterTest extends TestCase
             $writer->addRows($dataRows);
             $this->fail('Exception should have been thrown');
         } catch (SpoutException $e) {
-            $this->assertFileNotExists($fileName, 'Output file should have been deleted');
+            $this->assertFileDoesNotExist($fileName, 'Output file should have been deleted');
 
             $numFiles = iterator_count(new \FilesystemIterator($tempFolderPath, \FilesystemIterator::SKIP_DOTS));
             $this->assertEquals(0, $numFiles, 'All temp files should have been deleted');