Issue #3044678 by Pancho: Fix coding style issues all around backend plugins

src/Component/Utility/FillPdf.php

src/Controller/HandlePdfController.php

src/FillPdfAdminFormHelperInterface.php

src/FillPdfBackendManager.php

src/FillPdfBackendPluginInterface.php

src/Form/FillPdfOverviewForm.php

src/Form/FillPdfSettingsForm.php

src/InputHelper.php

src/Plugin/BackendService/LocalService.php

src/Plugin/BackendServiceInterface.php

src/Plugin/FillPdfBackend/FillPdfServiceFillPdfBackend.php

src/Plugin/FillPdfBackend/LocalFillPdfBackend.php

src/Plugin/FillPdfBackend/LocalService.php

src/Plugin/FillPdfBackend/PdftkFillPdfBackend.php

src/Service/FillPdfAdminFormHelper.php

...s/modules/fillpdf_test/src/Plugin/BackendService/Test.php

...pdf_test/src/Plugin/FillPdfBackend/TestFillPdfBackend.php